WASHINGTON (AP) — More than 68,000 illegally trafficked firearms in the U.S. came through unlicensed dealers who aren't required to perform background checks over a five-year period, according to new data released Thursday by the Bureau of Alcohol, Tobacco Firearms and Explosives.

That represents 54% of the illegally trafficked firearms in the U.S. between 2017 and 2021, Justice Department officials said. The guns were used in 368 shooting cases, which are harder to investigate because unlicensed dealers aren't required to keep records of their sales that could allow federal agents to trace the weapon back to the original buyer, said ATF Director Steve Dettelbach.

The report ordered by Attorney General Merrick Garland is the first in-depth analysis of firearm trafficking investigations in more than 20 years. It examined more than 9,700 closed ATF firearm trafficking investigations that began between 2017 and 2021. Firearms trafficking is when guns are purposely moved into the illegal market for a criminal purpose or possession.

The second-highest share of firearm-trafficking cases investigated by ATF was straw purchases, when someone buys a gun for a person who can't get it legally themselves.

The report also shows that the recipients of trafficked firearms were people who had previously been convicted of a felony in almost 60 percent of the cases in which investigators were able to identify the background of the recipient. Furthermore, trafficked firearms were used to commit additional crimes in almost 25 percent of the cases, Dettelbach said. That includes more than 260 murders and more than 220 attempted murders, according to the report.

"The data shows, therefore, that those who illegally traffic firearms whether its out of a trunk, at a gun show or online are responsible for real violence in this nation," Dettelbach said. "In short, you can't illegally help to arm violent people and not be responsible for the violence that follows," he said.

The report found the average number of guns trafficked per case was 16. People who got them through unlicensed dealers bought 20 weapons on average, compared to 11 guns for straw buyers, according to the report.

The Biden administration has separately proposed a rule that would require thousands more gun sellers to get licensed and run background checks. The Justice Department says it's aimed at sellers who are in the business of firearm sales, but the proposal quickly drew protest from gun-rights groups who contend it could ensnare regular people who sometimes sell their own guns.

The rule, which has not yet been finalized, is estimated to affect 24,500 to 328,000 sellers. During the five years documented in the report, 3,400 unlicensed dealers were investigated by the ATF.