So-called 3D-printed ghost guns are untraceable firearms that can be assembled at home. But cutting edge work from a forensic expert in California and researchers at the University of Oklahoma may soon show investigators can trace a 3D printed object to the specific printer that made it.Weapons manufactured using 3D printers have been a subject of Biden-era legislation and recent Supreme Court scrutiny. Its possible to download the blueprints for a firearm and build it in your home. Theres no serial number to track and no store to scrutinize your purchase. Luigi Mangione used a ghost gun to allegedly assassinate United Healthcare CEO Brian Thompson.Kirk Garrison, a forensics expert who works for the San Bernardino Sheriffs department, told 404 Media hes had early success matching 3D printed objects to the machines that made them. Garrison said his comments represent his own views and not those of the San Bernardino Sheriffs department. He also cautioned that what hes doing is in its infancy and it might be years before authorities can reliably match a gun to the machine that made it, if they can do it at all.In 2018, Garrison started seeing a lot of 3D printed gun parts in his work at the Sheriffs department. It was mostly 80% kits and automatic conversion kits, small 3D printed pieces of plastic that turn a semiautomatic pistol into an automatic one. Then he got his first case with a fully 3D printed gun frame. Thats when I was like, We might need to know a little bit more about this now if were actually going to be seeing this stuff and potentially have to testify to it, he told 404 Media.A few years later Garrison attended a conference for forensic examiners in Atalanta and caught a talk by FBI lab tech Corey Scott. Scott had been 3D printing novelty items and noticed something. He was just like, Hey, I noticed on these 3D printed items, theres these marks, but we was like: Im not actually a firearms or toolmark examiner.A toolmark is a consistent scratch or impression a harder object leaves on a softer one. A screwdriver may produce the same scratches in the head of every screw it touches. A pair of bolt cutters will scratch up a length of chain in the same way every time. Matching tools to the objects they interacted with is one of the bedmarks of forensic science and its something Garrison is an expert in.So the question was: do 3D printers leave behind consistent toolmarks on the objects they make? When he got back to his San Bernardino lab following the conference, Garrison put the 3D printed weapon frame under the microscope. He noticed that the manufacturing process had left stria, or scratch marks, behind. If a 3D printer left behind the same pattern of stria on everything it printed then it might be possible to match a printer to an object it printed.From there, Garrison started printing simple blocks at home on his own 3D printer. Hed take them into the lab on his own time and examine them under a microscope. Thats when I started seeing some of the consistency on two separate printed things, he said. It was too early to tell, and its still too early to tell, but individual printers might leave behind unique toolmarks on every object they print.A page from 'An exploratory study of topographical signatures within 3D fused deposition modelling using Polylactic Acid (PLA) filament.'Most 3D printers work by heating up a filamentoften, but not always, plasticand extruding it through a metal nozzle. The nozzle puts down hundreds, or even thousands, of layers of the heated plastic to form a solid object. Each individual level of the print is called the print line. So on the firearm, Im seeing from the trigger guardmaybe print line 200and the top of the magazine wellprint line 400the marks are staying consistent, Garrison said.It was an exciting discovery but it also wouldnt be admissible as evidence in a criminal trial. Despite the promise that we may one day be able to match a printer to the object that made it, Garrison stressed that the work was in its very early days and that it would take years, perhaps even a decade, of science to work out the truth of toolmarks and 3D printers.He was also studying this on his own time and still had a full caseload with the Sheriffs department. Garrison published a study about his results in the Forensics Science International that he co-authored with researcher Steven Pavlovich, but he knew there was more to do. Ive always been like, Hey, someone who works at a university who gets paid to do this, you should totally do this right now, he said.Enter Eric Law, an Assistant Professor at the University of Central Oklahoma Forensic Science Institute, and his graduate student Cooper Blair. Along with Garrison, the pair are the authors of a forthcoming research paper about the phenomenon of toolmarks in 3D printed objects. Once published, itll be the first of its kind.Law and Blairs focus is narrow. So if we had a single printer and we had multiple nozzles, can we tell the difference between something printed on each of those different nozzles? And also, if we have different print bed surfaces, can we differentiate those print bed surfaces and tell what object was printed on which? Law told 404 Media.The nozzles used in 3D printing are often, but not always, made of metal and printed onto a strip of material thats called a print sheet or print bed. They studied print sheets first. Not all sheets are the same, some are smooth, some are textured, and they come in a variety of different materials. So we looked at textured, because we figured if there's some texture to it, those characteristics might reproduce on the plastic, and might let us do that comparison a bit easier, Law said. So I looked at texture print beds, and we could differentiate those 100% of the time. Meaning that, both by eye and using a computer, his team could match an object to the sheet it was printed on.Its a promising early finding. The problem we get into there is we're looking at a specific area on the print bed, so you have to print something on the exact same region, because every area on that print bed is different, Law said. If we print something right in the center and then print that same object in the top right corner, those would be different from each other. So it has to be in the same location, which complicates things a little bit.He pointed to Glock switches, the conversion kits that turn a pistol into an automatic weapon. Those are pretty small and on a 3D print bed you could align a bunch of those and print them all at once, he said. Which is what you would do to produce as many as you can, as quickly as you can. If you had two of those they might look like they're from different printers, but they might have just been from different sections of the same printer.Print sheets can also move between printers and can be easily discarded. Knowing that a Glock switch was printed out on a particular sheet is not a smoking gun. So it shows promise. But there's a lot of potential issues too, Law said.Law and Blair succeeded in matching nozzles to printed objects in their study, but the results werent as promising as the print sheets. Law said the nozzle match rate was correct about 75 percent of the time. The algorithm could identify the correct nozzles, probably a little bit less than that with just visual examination, he said. It still shows promise, but is a bit more challenging.There are other issues too. All of Law and Blairs tests were done with one kind of 3D printera Prusa MK4S. Theres hundreds of different devices on the market that all behave differently. Law also pointed out that brass nozzles warp over time themselves and may produce different results after hundreds of prints and that different nozzles made from different materials may work very differently. Law would also want an examiner rate studya formal scientific inquiry into false positives and examiner bias.Theres a lot of promise in what weve seen but theres also a lot of questions still. Different nozzles, different print beds, how easy it is to swap those and whether they change, Law said. He would not, at this point, be willing to testify in a criminal case as an expert on 3D printed forensics.Garrison also said he wouldnt be comfortable using any of this in a court but he was still excited. Even if it doesnt work, and this is not a possibility, we still found out new information. Id be just as happy with that. Hey cool, I was involved in finding out that you cant do this, he said.