Testing the upside-down gun theory: Part 4

"20/20" asked forensic scientist Peter Diaczuk to try to recreate the way authorities said O'Connell could have shot herself.
8:19 | 06/02/18

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Transcript for Testing the upside-down gun theory: Part 4
Reporter: 1,000esm thiconic St. Augustine lighthouse, a Pennsylvania forensic scientist is trying to shed some lionious death of Michelle o'col. Today we've come to roll call, an indoor shooting range Scranton, to examine two key points in this invga using Dr. Bus theory, want to see where the shell casings may land when the GU upside-down, and see whether an -down gun, once D, could have caused the injury above Michelle's eye. Sheriff shoar has been critical her crime SNE reconstr by the fd. So today we are going inside. We've created a replica of the scene ban scene photos. We use anequin for the purposes of this test. And case this the layout of the bedroom, door right here, people came in out of the room this way. We ask professor Peter diaczuk, a forensic scientist at Penn state, to conduct a firing test for us. Back in 2013 diaczuk reviewed the case for a "frontli"new York Times" investigation. Professo wdo you kno the gun that was used in this case? I am familiar with the, with the firearm. Reporter: Hyou fired gun before? Many, many S. Rorter: In those initial set of photographs takthe there's no visible sign of any shell casingsun Michelle's body, but two WER later clearly photograph to her left in this back corner O the bedroom. This bag right here was where a shcasing wasndother one found about 18 inches away The location of the spent cases is to be dep upon several factors. One of the mosofs the position of the firearm when it was fired. Reporter: We'e the me make and model of the gun thir fatal shot plus live rounds. To keep those rounds under control, we're goio be firing into ballistic gelatin. Reporter: At the end of the day, what is tyou want to find out? We're going to see with our documentation where the cartridge cases will land depending on the position of the arm. Reporter: Yougoing to B lding that gun upside down and firing it. Why is that? That's because we are going toreplicating the position of the firearmdictated B the repo and the intervi of the medical examiners. Repor all right. Be safe. We walk about 15 yards to observe the firing tend bulletproof glass. The mannequin is laying down The most likely set of circumstances is that she was two-handed and fired, ahen the bldoes exactlat is seen in the pies, and goes this way. S he's holding it? Upsiden is the way authorities say she held it. 3, 2, 1. At's loud. Reporter: Using high speed photogy, you can actual see the shell casing ejecting towards the right side of the mannn. That is on the opposite side of E the shell casings were found at then. See those casings coming this way? Eacme, this way. Again. Again. Where did authorities find thosecasings? On the other side, where the professor let's go talk to him. Okay, professor. W many shots did you just fire? I fired tentridges. Orter: So, we have nine over here, one right here under her knee That's correct. Reporter: So if sheyi down, ldg the gun like that, there's no way that if sired those shots that thoseings would over here? That's correct. It just don't conform to the yshe way the F is designed. Reporteriff shoar has an explanation. Those shell cacould have been inadvertently kick any one of nine first onders at sc so, let's take a look at this. Ring in roll owner Joseph Harris, a retired corporal with Scranton police department, who's worked hundreds of crimenes over S career. Based on what you see here, how possibleitt a casing that ends up on this Michelle ended up all the way over there. It's a far distance. Reporter: How likely is it CING would' been kickedm all the WER to there? I would say, not so likely. Reporter: Ten bullets ready in the CLI we reset. Now you see that the dummy is sitting up. That's the other way that authorities sain their mind,that michemihave killed herself. Casings coming this way. Lolike almost eversing came to the right of mlle. Next, we position the mannequin kneeling on another possibility of how mile might have sh herself. Ten more shots, and this time we learn the sheriff is right aboutone thing. Objects in the room can play a role in where casings land. So, you've now fired 30 shots, three different positions. Based on the way you shot, what are the chances that a shell casing would end up over here? Incredibly unlikely. Reporter: Tbeing right here, perhaps ce been kicked there, but is there a contrary argument to that? He contrary argument would F the direction of travel of the responders is tod to the victim, and the dos that part of the building, then would be moikely that it uld be kicked in same direction that they're travg,hich is tohe right of the victim. Rep while our T raises questions about Dr. Bulic's de-down gun theory, it does noovor disprove that it suicide or micide. Michelleamily is convinced someone else was holding the gun. Hat would happenthe professor stood over Michelle, and shn towards not thn turned upside- anymore, but just straight away. The cartridge S were ejeed from the firearm and create pattern on the victim's left. Rter: What's intein is there's a shell casing all the way he, and Thi just a couple of, what, maybe up of inches away from where one of the cgs was found there. Is it more likely that she was impacted by this E way that the police described it, or in the way that family bees, which is Jeremy shot her? Our expentation clearly reveals that the family's scenario ie more likely scenario. Reporter: We showed LE's mom pty the results of our firing test. Why wouldn't the sheriff take the time to do a test?I see it and ihe did not Kise, and thathat I feel. Reporte feelings are not facts. There's one thing wed to examine. Remember Michelle's eyelid injury? Dr. Bulic toldfrontline" it was the tac light thatd thnjury. The only sound, solid forensic exption is that the gun was upside-down and the tactical laused that. Reporter: To the experts out E that say thaen you fire a gun like that, that the guuld not got instead would go back, that it would recoil backwards. Not so, says jerattorney. What do I say to E experts, and named their name, I'd say you er take the gun out and, test it before you say that. Reporteut that's exactly what professor diaczuk did using highpeed photography you can clearly see the gun going backwards, and flying away from what would have been Michelle's face. Firdo not go ard upon firing. It's again laws of physics. If you measure the arc of the C light, and you measure the arc of the wound, they are Y consistent. The tac L may have done that in an antemm event, but did not happen at the moment that the firearm was F. Repor in other words, diaczuk sa it could have happened before the fatal gunshot. But special prosecutors embraced bulic's theory, and determ there not enough evidence to charge Jeremy with a crime. So if that gunshot didn't cause the eye injury, what did?

This transcript has been automatically generated and may not be 100% accurate.

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