Facing what appears to be an extensive body of evidence, attorneys for accused Charlie Kirk killer Tyler Robinson signaled in a new filing they may call an ATF agent to the stand — if his inconclusive ballistic findings undercut the case.

"Regarding the firearm evidence, the defense has been provided with an ATF summary report which indicates that the ATF was unable to identify the bullet recovered at autopsy to the rifle allegedly tied to Mr. Robinson," Robinson's attorneys wrote in a motion to vacate or postpone their client's preliminary hearing. "Although the State has not indicated an intent to produce this report at the preliminary hearing, the defense may very well decide to offer the testimony of the ATF firearm analyst as exculpatory evidence."

The brief line appears to indicate that the defense doesn't think prosecutors can prove the bullet that killed Kirk came from the Mauser rifle recovered near the crime scene, which prosecutors allege had Robinson's DNA on it and had been given to him by his father.

But experts say inconclusive testing doesn't mean the match has been ruled out, and investigators have compiled other evidence in the case — including allegations that Robinson confessed or implied guilt to family and friends.

"It's not uncommon for a round that went through a human body, especially if it traveled through tissue, to say conclusively that it's tied to a specific firearm, because by nature the projectile is supposed to transfer all of its kinetic energy, and it often disintegrates into fragments and whatnot," said Bernard Zapor, a former ATF special agent in charge and faculty associate at Arizona State University. "The fact that it went through several bone structures, there was going to be very little left."

As a result, it would be unfair to conclude that the bullet hadn't been fired out of the rifle police recovered nearby, experts told Fox News Digital.

"Unable to identify is not the same as ruled out," said retired FBI supervisor agent Jason Pack. "That’s a finding of inconclusiveness, not exoneration."

Two law enforcement sources familiar with the investigation later told Fox News that the reason the ATF was unable to match the bullet to the rifle is because when the bullet impacted Kirk’s body it hit bone and broke on impact. 

The forensic pathologist pulled fragments out of Kirk’s body, not an intact bullet. The sources said it is impossible to do ballistics analysis on a bullet fragment.

The testing did positively confirm that the spent shell casing found at the scene matched the suspected murder weapon.

"That’s a pretty significant piece of evidence for the prosecution," Pack said. "And pretty damning for the defense."

According to the Utah County Prosecutor's Office, Robinson arrived at Utah Valley University around 11:51 a.m. on Sept. 10, 2025, the day of Kirk's murder. They allege he walked in through a tunnel under Campus Drive and climbed a staircase to the rooftop of the Losee Center, which overlooks the courtyard where Kirk was speaking.

At 12:20 p.m., a single bullet struck Kirk in the neck. Prosecutors have said campus police found marks left behind on the gravel rooftop moments after the shooting "consistent with a sniper having lain [there] — impressions in the gravel potentially left by the elbows, knees and feet of a person in a prone shooting position."

Police later found the alleged murder weapon wrapped in a towel and hidden in a small patch of woods just off campus. Crime lab testing allegedly found DNA consistent with Robinson's on the gun, on the towel and on three of the four rounds inside.

"To me that’s not problematic, but obviously the defense is gonna make big hay out of that, because they want to have the firearm removed from the case," Zapor said. "If the shell casing has his DNA on it, [prosecutors] are solid."

There are a number of reasons why the fired bullet may not have been a conclusive match, experts told Fox News Digital, and additional lab testing is pending.

"Nobody outside the ATF lab knows why they couldn’t make the match yet because the defense still hasn’t received the underlying case file and protocols," Pack told Fox News Digital. "Was the bullet too damaged? Was it a methodology question? That answer isn’t in the record right now."

Other evidence in the case includes a text message conversation prosecutors allege Robinson had with his roommate and romantic partner, Lance Twiggs, after the shooting.

Robinson allegedly discussed wanting to retrieve a rifle before returning home from Orem, which is about a four-hour drive from where he lived in Washington County. Twiggs is cooperating with investigators and has not been charged with a crime.

In another message, Robinson allegedly wrote he was "stuck in Orem for a little while longer yet. Shouldn't be long until I can come home, but I gotta grab my rifle still. To be honest I had hoped to keep this secret till I died of old age. I am sorry to involve you."

"You weren't the one who did it right????" the roommate texted back.

"I am, I'm sorry," Robinson allegedly replied.

Robinson faces a top charge of aggravated murder, which carries the potential death penalty upon conviction. He is also accused of felony discharge of a firearm causing serious bodily injury, obstruction of justice, witness tampering and committing a violent offense in the presence of a child.

While Robinson's defense is seeking to put off his preliminary hearing for another six months, Kirk's widow, Erika Kirk, has filed a notice with the court invoking her right to seek a speedy trial as a victim under Utah law.

Kirk, the founder of Turning Point USA, was speaking at an event as part of a national speaking tour and sponsored by the campus chapter of the organization, which promotes conservative principles among students across the country.

He was a married father of two.

Fox News' Jake Gibson contributed to this report.