Lawyers for the man charged with killing conservative activist Charlie Kirk have asked to delay a preliminary hearing scheduled in May, saying they need time to review an enormous amount of material and a bullet analysis that could contribute to his defense.

Tyler Robinson’s defense team said in recent court filings that an analysis from the Bureau of Alcohol, Tobacco, Firearms and Explosives, a federal law enforcement agency, could not conclusively connect a bullet fragment recovered during an autopsy to the rifle found near the scene. The FBI is running additional tests, according to court documents.

The agency’s report has been kept private, but attorneys have cited snippets in other public filings that say the results were inconclusive.

The success of a forensic ballistics analysis largely depends on the size and condition of the bullet fragments. Experts are looking for unique, microscopic markings that are left on a bullet as it passes through the gun’s barrel. The scratches are like fingerprints in that no two firearms make identical markings.

The defense said in its motion that it may try to use the analysis to clear Robinson of blame during the preliminary hearing, while prosecutors aim to show they have enough evidence against him to proceed with a trial.

Prosecutors intend to seek the death penalty for Robinson, 22, who is charged with aggravated murder in the Sept. 10 shooting of the conservative activist on the Utah Valley University campus in Orem. Robinson has not yet entered a plea.

Prosecutors have said DNA consistent with Robinson’s was found on the trigger of the rifle, the fired cartridge casing and two unfired cartridges. Defense attorneys note that forensic reports indicate multiple people’s DNA was found on some items, which they say requires a more complex analysis.

Robinson reportedly texted his romantic partner that he targeted Kirk because he “had enough of his hatred,” prosecutors have said.

Robinson is due back in court April 17 for a hearing on a defense motion to ban cameras from the courtroom.