In the chaotic hours following the murder of Charlie Kirk, investigators found a firearm in a forested area near the crime scene in northern Utah. Tracing this weapon presented a significant challenge for federal agents.

This firearm was an old German-made rifle, originally designed for military use in the World Wars, according to several law enforcement sources involved in the case. The rifle was so ancient that it might have been imported into the U.S. before the 1960s legislation required guns to have serial numbers or identifying marks for tracing.

There are believed to be millions of such weapons in homes across America.

Luckily, investigators were able to identify the suspected shooter through other channels—his family, who persuaded him to turn himself in. However, the suspected use of such an outdated firearm has created concern among former federal agents about similar weapons being sought by potential assassins due to their power, accuracy, and difficulty to trace.

“Outside of the high-level security given to the president, defending against threats like this is nearly impossible,” remarked Scott Sweetow, a former official with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The alleged shooter, Tyler Robinson, surrendered to law enforcement a day after Kirk was killed at an event on the Utah Valley University campus. In text messages cited by prosecutors, Robinson admitted to his roommate and romantic partner, who is transgender, that he killed Kirk because he “couldn’t take his hatred anymore.”

Robinson, aged 22, faces charges of aggravated murder along with six other offenses. Prosecutors describe him as having recently become more supportive of gay and trans rights.

A public memorial service for Kirk, the hugely popular but also polarizing founder of the conservative group Turning Point USA, will be held Sunday in Arizona.

A popular bolt-action rifle

Prosecutors identified the recovered gun as a Mauser model 98. The .30-06 caliber rifle, which fires cartridges slightly smaller than 8 millimeters, had belonged to Robinson’s grandfather, prosecutors said.

The authorities have not revealed how they believe Robinson’s grandfather obtained the gun or whether they were able to successfully trace it. An ATF spokesman declined to comment to NBC News.

The gun is a bolt-action rifle, which requires users to manually reload between shots. A person must pull the bolt to the rear and then move it forward in order to move a new round into place and fire again.

Following the wars in Europe, countless American GIs returned home with German-made Mausers. In subsequent years, Mauser rifles and other similar versions could be obtained simply by ordering them through the mail.

For decades, they have been favored by hunters who prize them for their durability, reliability and accuracy. With .30-06 rounds, they are ideal weapons to take down medium to large game like deer, elk and even bear.

But it is exceedingly rare for them to be used in crimes, according to former ATF gun agents and federal reports. Most gun violence in America is carried out with pistols, and semiautomatic AR-15-style rifles have been used to commit some of the nation’s deadliest mass shootings.

“A bolt-action rifle? You don’t see those guns used in violent crimes,” said Tim Sloan, a former ATF agent who retired as the head of its Mexico branch.

That’s because they can’t be as easily concealed as handguns and they’re not designed to spray a large volume of bullets at once, experts say.

But they are, in some ways, ideally suited for use in sniper-style killings. Because they are so ubiquitous and beloved by hunters and recreational shooters, a person toting such a rifle in a place like Utah would not arouse any suspicion.

And they are so well made that, with a decent scope, a person with basic firearm proficiency can strike a target from 150 to 200 yards away, roughly the distance of the shot fired by the Kirk assassin.

“You do not have to have gone to sniper school or have been any type of champion marksmen,” said Brian Greco, a retired NYPD officer who served in the counterterrorism unit and a Marine Corps veteran.

Despite their age, vintage Mauser rifles like the one authorities say was used to kill Kirk are widely available at firearm stores, pawn shops and gun shows, experts said. They can also be modified to accommodate different rounds, scopes and lighter stocks made of fiberglass.

“The basic, plain-Jane Mausers — you can go to any gun show in America and see these things for a few hundred dollars,” said Sweetow, the retired ATF official. “The heavily customized versions can go for thousands.”

Echoes of Kennedy assassination

The use of a decades-old, European military surplus rifle to kill Kirk echoes one of America’s most infamous crimes: the assassination of John F. Kennedy in 1963.

The killer, Lee Harvey Oswald, used a Carcano model 38, a bolt-action rifle originally designed in the late 19th century for the Italian military. He obtained it by mail order through an advertisement in a gun magazine.

The killing helped to usher in the Gun Control Act of 1968, which among other things required firearms to be sold by federally licensed dealers, to have markings to enable tracing, and for paperwork to be filled out identifying the buyer.

Gun control advocates have long been pushing for more restrictions, such as a ban on semiautomatic, military-style firearms like AR-15s. But they acknowledge that such a ban would not cover a bolt-action rifle like the Mauser 98.

“It’s a hunting rifle, and people hunt legitimately,” said Greg Lickenbrock — author of the book “Safe Gun Ownership for Dummies” and a senior firearms analyst for Everytown for Gun Safety, a gun-control group founded by Mike Bloomberg.

He said the use of this kind of gun in a violent crime highlights the gaps in law enforcement’s ability to track down the owners.

“This is a very obvious example of the tracing system having huge holes in it,” Lickenbrock added.

That view is shared even by some who believe strongly in the Second Amendment.

“I am definitely not a ‘more government’ type of person, but as someone who takes firearms safety seriously and has worked with firearms for 30 years through the Marine Corps, NYPD and now as an armed guard for a Long Island school district, I do feel there has to be accountability to these older and unserialized firearms,” Greco said.

“A vintage firearm in grandpa’s old trunk is cool,” he added, “but it also unfortunately gives access to a firearm to someone who may otherwise have not had access or could have been denied access.”