Authorities said the gun used in the killing of Charlie Kirk may be a decades-old, untraceable rifle, authorities have said.

Prosecutors and officials identified the gun as a Mauser Model 98 .30-06 caliber bolt-action rifle, a type originally manufactured in Germany for military use during both World Wars. It may predate U.S. laws enacted in 1968 following President John F. Kennedy’s assassination, which required firearms to carry serial numbers or other identifying marks — making it harder for law enforcement to trace.

Authorities said the firearm belonged to the grandfather of Tyler Robinson, the 22-year-old Utah man accused of assassinating Kirk. Police allege Robinson may have deliberately chosen the rifle because it was difficult to trace. 

Robinson himself appeared to echo that reasoning in a message to his roommate just hours after the assassination. 

ENGRAVINGS ON SHOOTER’S AMMO EXPOSES CHARLIE KIRK ASSASSINATION MOTIVE

“I’m worried what my old man would do if I don’t bring back grandpa’s rifle,” Robinson wrote, according to the New York Post. “IDK if it has a serial number but it wouldn’t trace to me. I’m worried about prints, I had to leave it in a bush where I changed outfits, didn’t have the ability or time to take it with.”

Court documents indicate that the Utah State Bureau of Investigation found DNA on the murder weapon that officials say is consistent with Robinson.

KIRK ASSASSIN USED GRANDFATHER’S RIFLE TO COMMIT ACT AND PRAISED GUN AFTERWARDS, AUTHORITIES ALLEGE

Vintage firearms may complicate future investigations due to how easily would-be assassins can obtain such powerful and hard-to-trace weaponry, with millions believed to be in homes across America. After the wars in Europe, countless American soldiers returned home with German-made Mausers. In the following years, Mauser rifles and other similar models could even be purchased through mail. 

WHAT IS A BOLT ACTION RIFLE? WHAT WE KNOW ABOUT THE GUN USED TO KILL CHARLIE KIRK

A report from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) notes that it is exceedingly rare for such rifles to be used in crimes. Most gun violence in America is carried out with pistols, while semiautomatic AR-15-style rifles have been used in some of the nation’s deadliest mass shootings.

ATF and the Federal Bureau of Investigation Salt Lake City did not immediately respond to a request for comment from Fox News Digital.