Striated toolmarks comparison and reporting methods: Review and perspectives.

Forensic toolmark examiners have been comparing features observed in toolmarks to help determine their source for over a century. However, in the past decade, the holistic process of comparing toolmarks and presenting findings in court have faced intense scrutiny. This paper provides a summary of the voiced criticisms, primarily concerning the scientific reliability and validity of the comparison methods employed by examiners and the conclusions they testify to. The focus of this review is specifically on the examination of striated toolmarks. We assess the comparison methods and reporting practices currently in use, while also delving deeper into research aligned with current recommendations, such as PCAST (The President's Council of Advisors on Science and Technology). Throughout the review, we examine both the strengths and weaknesses of existing practices, aiming to assist practitioners in identifying key research needs and addressing the concerns raised by critics. By doing so, we seek to enhance the credibility and effectiveness of toolmark analysis in the field of forensic science.

Automated comparison and evaluation of striated cutting plier toolmarks on metal wires.

Toolmarks examination validity and subjectivity have come under scrutiny. This research focuses on the case of cutting plier marks. This paper presents an automatic comparison method and assesses its performance. It is designed to assign a weight to the forensic evidence (i.e, a comparison between toolmarks) with a likelihood ratio (LR). 3D topographies are acquired and treated to be compared using a set of correlation metrics. A machine learning algorithm combines comparison metrics and enables LR computation. Pliers of various brands and models were used to study the variability both within and between tools. We explained why the specific zone (area along the blade) has to be chosen to build the within-source variability and how the between-source variability can be built in different scenarios. Misleading evidence rates between 0 % and 4 % have been measured and it demonstrates the accuracy of the method when applied on the pliers used.

The 2 stages of cartridge primer toolmark production and the implied impact of cartridge manufacturing tolerances.

Many methods of ballistic toolmark comparison rely upon comparison using 2D greyscale imaging. However, newly emerging analysis methods such as areal surface analysis now utilise an extra dimension of measurement allowing the surface heights/depths of unique toolmark features to be recorded in a densely populated (x,y,z) array for a 3D/areal quantitative comparative analysis. Due to this step change, the colloquialism in referring to the crater produced at the centre of the primer during firing as a "firing pin impression" has become a misnomer, leading some to believe that this toolmark is produced via a single process, where the critical variable is the condition of the firing pin. Furthermore, current forensic ballistic methodology relies on the microscopic differences between individual fired bullets and cartridge cases produced as a result of the manufacturing process of a particular firearm, in this case "matched toolmarks" confirm a ballistic match to a specific firearm. However, very rarely is it considered that the ammunition itself possesses minute differences produced during manufacture that could affect the ballistic match efficacy. This study examines the discharge process of conventional centrefire ammunition and concludes that the unique toolmarks upon the cartridge primer are definitively produced in two defined stages. This conclusion suggests that the factory loading and quality control tolerances of the cartridge itself should now be considered to be a more significant contributing factor to the production of cartridge primer toolmarks than has previously been accepted.

Saw marks in bone: A preliminary empirical study to inform decision making and best practice.

Forensic toolmark examiners compare marks between those observed on an item/surface and those made by a reference implement, such as a particular tool or weapon, to provide an opinion of the likelihood of common origin. It is widely accepted that such comparison opinions need to be underpinned by empirical research, and this study aimed to add to the knowledge base relied upon when developing and comparing saw marks in bone, a substrate encountered in body dismemberment cases. Porcine bones were used as a human proxy; they were either fresh with residual soft tissue and bodily fluids present ('wet') to replicate dismembered bones shortly post-mortem, or processed to remove soft tissue and moisture content ('dry') to represent cases of dismemberment after an extended period of decomposition and exposure. The bones were cut using one implement of each of five classes: hand saw, mitre saw, reciprocating saw, oscillating saw, and serrated knife. They were cut, either completely through (except for serrated knife), giving two surfaces per cut to examine, or to a depth up to 3 mm (false starts). Five replicates per combination of bone condition, saw, and cut type gave 130 bone samples. These were then cleaned and cast using Isomark Silicone Polymer Compound or Mikrosil, giving 260 cast samples. All bone and cast samples were photographed, examined for various class characteristic markers, and specific markers measured. No significant differences between Isomark and Mikrosil casts were observed when compared side-by-side, demonstrating suitability of both materials for casting of saw marks on bone. Although saw marks presented more class characteristic markers on dry than wet bones, calculations of tooth distances and measurements of kerf width (KW) from marks did not significantly differ between bone conditions, with exception of the reciprocating saw that produced false start marks with significantly larger minimum KW on wet than dry samples. Further analysis supported that tooth distances on marks made by hand and oscillating saws are sufficiently accurate for the determination of saw teeth per inch (TPI). However, one tooth distance on marks made by reciprocating saws did not accurately represent TPI. Finally, examination of presence or absence of class characteristic markers on each saw mark demonstrated consistent variation between saw classes. These results enabled the development of exclusion-based decision trees, and a reference database (available on request), for use by toolmark examiners in their evaluation of saw types based on class characteristic markers observed in cut bone.

Assessing the thermal stability of toolmarks casting materials through optical comparison microscopy, virtual comparison microscopy and quantitative similarity scores.

Toolmarks, particularly those found on bulky, inaccessible or immovable items, can be recovered by casting. To allow for subsequent comparative examinations, the casting material, typically polysiloxanes or silicones, must be able to capture and preserve fine details within a toolmark accurately. To study the stability of such details after exposure to heat, toolmark casts were heated at either 60 °C for 2 h, or 90 °C for 1 h. These casts were subsequently compared to casts that had not been exposed to heat, using traditional optical comparison microscopy, as well as virtual comparison microscopy. Digitised toolmark signatures were also extracted from the casts and compared pairwise to obtain quantitative similarity scores based on cross-correlation, consecutive matching striae and Mann-Whitney U-statistic. Our results show that the fine surface details captured on all four commercial toolmark casting materials tested herein remained stable after exposure to heat. This study shows that the above heating protocols are viable viral inactivation methods for toolmark casts that are potentially contaminated with human coronaviruses, such as SARS-CoV-2. Our findings also apply to other scenarios, such as for casts that were left in a vehicle parked under the sun.

Validating strength-of-support conclusion scales for fingerprint, footwear, and toolmark impressions.

In the pattern comparison disciplines such as fingerprints, footwear, and toolmarks, the results of a comparison are communicated by examiners in the form of categorical conclusions such as Identification or Exclusion. These statements have been criticized as requiring knowledge of prior probabilities by the examiners and being overinterpreted by laypersons. Alternative statements based on strength-of-support language have been proposed. The current study compares traditional conclusion scales against strength-of-support scales to determine how these new statements might be used by examiners in casework. Each participant completed 60 comparisons within their discipline, which were designed to approximate casework conditions, using either a traditional or a strength-of-support conclusion scale. The scale used on each trial was randomly assigned, and participants knew the scale for that trial as they began the comparison. Fingerprint examiners were much less likely to use Extremely Strong Support for Common Source than Identification. Footwear examiners treated the traditional and strength-of-support scales similarly, but toolmark examiners were much less likely to use Extremely Strong Support for Common Source than Identification, similar to fingerprint examiners. A separate group of fingerprint examiners used Identification less often when an expanded scale was available. The results demonstrate that expanded scales may result in the highest conclusion category being used less often by examiners when other alternatives are possible, and the term "extremely strong support" may introduce risk aversion on the part of examiners.

Surveying practicing firearm examiners.

A sample (n = 79) of practicing firearm and toolmark examiners was queried about casework as well as their views about the potential role that statistics might play in future firearm examinations and expert witness testimony. Principal findings include: The modal response for time spent conducting bullet examinations is 2-4 hours, and the modal response for cartridge casings is 1-2 hours. The average participant (median) makes an identification in 65% of casework, makes an elimination in 12% of casework, and reports that the examination was inconclusive in 20% of casework calls. The vast majority of examiners work at laboratories that permit eliminations when class characteristics agree. The reported industry-wide false positive error rate is 1%, though very few participants could name a study or give a citation for their reported estimate. Qualitative responses about the potential role of statistics were mixed.

Accurate prediction of saw blade thicknesses from false start measurements.

BACKGROUND: False start analysis is the examination of incomplete saw marks created on bone in an effort to establish information on the saw that created them. The present study aims to use quantitative data from micro-CT cross-sections to predict the thickness of the saw blade used to create the mark. Random forest statistical models are utilised for prediction to present a methodology that is useful to both forensic researchers and practitioners. METHOD: 340 false starts were created on 32 fleshed cadaveric leg bones by 38 saws of various classes. False starts were micro-CT scanned and seven measurements taken digitally. A regression random forest model was produced from the measurement data of all saws to predict the saw blade thickness from false starts with an unknown class. A further model was created, consisting of three random forests, to predict the saw blade thickness when the class of the saw is known. The predictive capability of the models was tested using a second sample of data, consisting of measurements taken from a further 17 false starts created randomly selected saws from the 38 in the experiment. RESULTS: Random forest models were able to accurately predict up to 100% of saw blade thicknesses for both samples of false starts. CONCLUSION: This study demonstrates the applicability of random forest statistical regression models for reliable prediction of saw blade thicknesses from false start data. The methodology proposed enables prediction of saw blade thickness from empirical data and offers a significant step towards reduced subjectivity and database formation in false start analysis. Application of this methodology to false start analysis, with a more complete database, will allow complementary results to current analysis techniques to provide more information on the saw used in dismemberment casework.

A novel method for linking between a 3D printer and printed objects using toolmark comparison techniques.

Rapid advances and decreased production costs in 3D printing (3DP) have resulted in its accelerated implementation in criminal activities. Fused Deposition Modeling (FDM (3DP and Polylactic Acid (PLA) filament were chosen for the current research because they are widely used in commodity 3DP, particularly in documented criminal activities. This study shows how specific features of 3DP along with classical toolmark comparison techniques using Stereo-Microscopy and Comparison Microscopy can be used to link between two 3DP objects as well as between a printed object and a suspected 3D printer. Links are determined based on random fine marks found on the 3DP's heated stage (bed) that are replicated to the base face of the printed object. Melted filament that extrudes from the nozzle of the FDM 3DP constructs the base face of the printed object. This melted filament functions as a "casting material" after it cools down and solidifies, enabling replication of the fine marks. The observed resolution of these marks is as high as the resolution of casting material dedicated for toolmark replications. Overall, this study demonstrates a novel forensic method based on toolmark comparison for linking between a 3D printer and its printed objects.

Friction Marks of Stabbing Tools in Tires.

When receiving a stabbed tire for examination, forensic toolmark examiners can determine whether a suspect tool was used in a specific crime based on class-characteristics and individual-characteristics marks that have been left by the tool on the tire. This study discusses friction marks and their forensic value during the examination of a punctured tire. The term friction mark refers to the noticeable mark around the penetration area on a tire's surface. Tires designed to create high friction when contacting a road. Due to this design, friction is created between the stabbing tool shank and the sides of the hole. As a result of this friction, the shank of the stabbing tool wears the outer layer of tire around the hole. This leaves a friction mark whose general shape reflects the cross-sectional shape of the stabbing tool's shank. This phenomenon was observed and named by Locke (7) in his evaluation of tire puncture marks with knives. This article demonstrates the same phenomenon with other types of stabbing tools. Test stabs were produced with different tools representing a variety of cross-sectional shapes of shanks, and the resulting friction marks were photo-documented and discussed. Correlations between the various cross-sectional shapes and their corresponding friction marks are shown. Based on friction mark examination, the examiner: (i) can infer suspect tool shank cross-sectional shape with the evaluation of the friction mark shape and (ii) can deduce the maximum dimensions of the shank. This examination simplifies and accelerates the forensic comparison procedure and the investigation time.

Effect of Cutting with an Abrasive Saw on Swarf Composition.

Crime scene investigators are often asked to examine swarf from break-in sites and compare it with swarf found on a suspect. Occasionally, elemental composition of swarf from these two sources does not match. It is generally assumed that cutting metal does not induce changes in chemical composition. However, this study shows that, if a matrix contains iron, chromium, and nickel, composition of swarf produced from cutting the matrix with an abrasive saw may indeed alter. In this study, we cut a stainless steel sheet using a standard cutoff saw. Swarf was sorted and chemically analyzed by EDS/SEM, and the following relationships between chip morphology and modified composition were discovered: elongated chips remained unchanged; changes in the irregular chips appeared to be random, possibly because they are formed in a number of different ways; composition of spherical chips changed with size.

Micro-CT for saw mark analysis on human bone.

In toolmark analysis, microscopy techniques, such as micro-CT, are used to visualise and measure toolmarks left on bones by a tool. In dismemberment cases, properties such as the width of the saw mark can provide cues to which tool was used by the culprit. The aim of the current study was to establish whether; (i) micro-CT is an appropriate imaging technique for saw mark analysis, (ii) toolmarks statistically differ when created with different tools, (iii) toolmark width can predict tool blade width, and (iv) toolmarks differ if created under different methodological conditions. Across two experiments, 270 saw marks were created using eight tools with either a controlled or free saw action on either fleshed or defleshed human long bone. Toolmarks were micro-CT scanned and seven toolmark properties were categorised or measured by two independent raters. The current study found that; (i) micro-CT was found to be a powerful and reliable imaging method for the visualisation and measurement of saw mark properties, (ii) toolmark properties differed significantly within and between various methodological conditions (p<.001) when created by eight different tools, (iii) a regression model developed using toolmark widths from Experiment 2 overall predicted 94% of tool widths in Experiment 1, and iv) methodological factors such as tissue presence and saw action significantly and inconsistently influenced toolmark properties for different tools. The study further validates the use of mirco-CT for saw mark analysis and demonstrates the potential of using toolmark properties to determine the tool used in cases of dismemberment. Given the effects that methodological factors such as tissue presence can have on toolmark properties, future studies should use experimental set ups with fleshed human tissue and use a free saw action.

Improve Toolmarks' Impressions in Soft Wax.

When the forensic toolmarks laboratory receives for examination and comparison a tool that is suspected of having been involved in a crime, the expert performs tests designed to determine whether or not the specific tool generates the same toolmarks as those found at the crime scene. This is performed by testing tool striation on a piece of soft metal, such as lead, and examining the marks left by the tool. Studies have shown that wax may be an optimal material for this purpose. This study examines the use of wax at different temperatures and shows that quality of results is better when the wax is cooled (recommended temperature is -18°C). At this temperature, the wax is flexible enough but does not smear and is not sticky. This makes the obtained marks clearer and of better quality.

Toolmarks made by lathe chuck jaws.

This paper presents a forensic method to evidentially tie a workpiece with a specific lathe. Examining using this method can prove or exclude a connection between the two. The importance of this method is mostly due to the growing trend among lawbreakers of manufacturing improvised firearm parts using machining processes. This method is based on comparing jaw impressions made by the chuck on a workpiece.

Development of a Mobile Toolmark Characterization/Comparison System.

Since the development of the striagraph, various attempts have been made to enhance forensic investigation through the use of measuring and imaging equipment. This study describes the development of a prototype system employing an easy-to-use software interface designed to provide forensic examiners with the ability to measure topography of a toolmarked surface and then conduct various comparisons using a statistical algorithm. Acquisition of the data is carried out using a portable 3D optical profilometer, and comparison of the resulting data files is made using software named "MANTIS" (Mark and Tool Inspection Suite). The system has been tested on laboratory-produced markings that include fully striated marks (e.g., screwdriver markings), quasistriated markings produced by shear-cut pliers, impression marks left by chisels, rifling marks on bullets, and cut marks produced by knives. Using the system, an examiner has the potential to (i) visually compare two toolmarked surfaces in a manner similar to a comparison microscope and (ii) use the quantitative information embedded within the acquired data to obtain an objective statistical comparison of the data files. This study shows that, based on the results from laboratory samples, the system has great potential for aiding examiners in conducting comparisons of toolmarks.

Virtual and simulated striated toolmarks for forensic applications.

Large numbers of experimental toolmarks of screwdrivers are often required in casework of toolmark examiners and in research environments alike, to be able to recover the angle of attack of a crime scene mark and to determine statistically meaningful properties of toolmarks respectively. However, in practice the number of marks is limited by the time needed to create them. In this article, we present an approach to predict how a striated mark of a particular tool would look like, using 3D surface datasets of screwdrivers. We compare these virtual toolmarks qualitatively and quantitatively with real experimental marks in wax and show that they are very similar. In addition we study toolmark similarity, dependent on the angle of attack, with a very high angular resolution of 1°. The results show that for the tested type of screwdriver, our toolmark comparison framework yields known match similarity scores that are above the mean known non-match similarity scores, even for known match differences in angle of attack of up to 40°. In addition we demonstrate an approach to automatically recover the angle of attack of an experimental toolmark and experiments yield high accuracy and precision of 0.618 ± 4.179°. Furthermore, we present a strategy to study the structural elements of striated toolmarks using wavelet analysis, and show how to use the results to simulate realistic toolmarks.

Bolt Cutter Blade's Imprint in Toolmarks Examination.

Bolt cutters are known as cutting tools which are used for cutting hard objects and materials, such as padlocks and bars. Bolt cutter blades leave their imprint on the cut objects. When receiving a cut object from a crime scene, forensic toolmarks examiners can determine whether the suspected cutting tool was used in a specific crime or not based on class characteristic marks and individual marks that the bolt cutter blades leave on the cut object. The paper presents preliminary results of a study on ten bolt cutters and suggests a quick preliminary examination-the comparison between the blade thickness and the width of the imprint left by the tool on the cut object. Based on the comparison result, if there is not a match, the examiner can eliminate the feasibility of the use of the suspected cutting tool in a specific crime. This examination simplifies and accelerates the comparison procedure.

A physical match of a metallic chip found on a bolt cutters' blade.

Bolt cutters are known as devices which are used for cutting hard objects and rigid materials such as padlocks and bars. They are commonly used in instances of forced entries. In this case study, a bolt cutter was found in the car of two suspects in a grocery burglary. This study indicates how the presence of a small metallic chip found on a suspected bolt cutter can prove that the tool was used in the crime scene. During the initial examination, a metallic chip from the cut shackle padlock was found stuck to one of the bolt cutters' blades. By comparing the metallic chip's microscopic edge and the breaking (fracture) line of the padlock's shackle, a full physical match was noticed. We wish to report here how residue, even the smallest, can be used to link burglary tools to a crime scene with a high level of certainty.

Toolmark variability and quality depending on the fundamental parameters: Angle of attack, toolmark depth and substrate material.

The traditional way of visual toolmark comparison includes subjective judgments. Automated methods using computers are a possibility to render a comparison more objective, but they require the statistical properties, like the similarity and variability, of toolmarks to be determined quantitatively. Several parameters, that play a role during toolmark creation, are statistically analyzed in this article. We determined the same toolmark and the different toolmark similarity as well as variability of known matching toolmarks created in wax and compared the results with the similarity and variability of known non-matching toolmarks. In addition we studied the influence of the substrate materials wax and lead and the angle of attack on toolmark similarity and variability. Furthermore, we present an approach to determine toolmark quality, defined as how well structural details are preserved in the toolmark, to assist toolmark examiners in deciding, which structural details are reliable in a mark. We studied the influence of the substrate material, the angle of attack and the depth of a toolmark on the quality. The results show that for known matching toolmarks, the variability is very low within a toolmark and between toolmarks in wax, given that the parameters angle of attack and depth are held constant. Geometrical details are reliably represented down to 10-50µm and toolmark similarity is clearly higher than known non-matching similarities. The comparison of wax and lead shows that wax is a good alternative as a substrate material for experimental toolmarks, capable of reliably representing structural details down to 10-25µm. For finer details, lead is a better choice but might alter the original state of a tool. With increasing angle of attack, toolmark variability increases and toolmark quality decreases. Therefore it is advantageous to push the tool instead of pulling during toolmark creation for angles of attack above ≈45°. The quality also decreases with increasing toolmark depth, but only up to ≈300µm. Therefore toolmarks should be created as shallow as possible in the substrate material.

Identifying diagonal cutter marks on thin wires using 3D imaging.

We present work on matching 2-mm-thick wires using optical 3D imaging methods. Marks on such small surfaces are difficult to match using a comparison microscope as this 2D imaging method does not provide height data about the sample surface. Moreover, these 2D microscopy images may be affected by illumination. Hence, the reference and investigated sample should be present at the same time. We employed scanning white light interferometry and confocal microscopy to provide quantitative 3D profiles for reliable comparison of samples that are unavailable for simultaneous analysis. We show that 3D profiling offers a solution by allowing illumination-independent sample comparison. We correctly identified 74 of 80 profiles using consecutive matching striae (CMS) criteria, and we were able to match samples based on profiles measured using different 3D imaging devices. The results suggest that the used methods allow matching cutter marks on thin wires, which has been difficult previously.