Hundreds of firearms and gun parts have been stolen from the federal agency charged with gun enforcement, triggering a massive search for the missing weapons and demands for answers from U.S. Sen. Ron Johnson.

A former security guard working for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives has been charged with stealing the guns and firearms parts from an ATF facility in Martinsburg, West Virginia.

This week, Johnson wrote a letter to ATF Deputy Director Thomas Brandon with a series of questions on the theft, asking how many weapons were stolen, who was involved, if the guns and parts were used in crimes, if they have been recovered and others. Johnson, R-Wis., is chairman of the Committee on Homeland Security & Governmental Affairs. Sen. Gary Peters, D-Mich., the committee's ranking member, co-signed the letter.

They asked for answers from ATF by March 26.

The letter says the theft occurred at the ATF facility is Martinsburg, which handles the destruction of weapons no longer needed by ATF. The facility received an influx of guns and parts after the agency changed its standard-issue service weapons, it says.

The ATF lost about 600 gun parts worth $70,000, according to Johnson and Peters, who cited "certain sources" and news accounts. ATF refused to say how many guns and parts were stolen.

The case is similar to a series of failures and carelessness in ATF storefront operations in Milwaukee and nationwide, documented in a Journal Sentinel investigation. Those operations were intended to snare criminals selling guns and drugs but were fraught with problems including:

ATF-owned guns, including a machine gun, were stolen and the fully automatic weapon was never recovered.

Agents used a mentally disabled man to promote the operation and then arrested him. The storefront was burglarized and the building was damaged, leaving the landlord with unpaid bills.

Agents grossly overpaid for guns, some of which had been purchased the same day from retailers.

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In the current case, Christopher Yates, a contract security guard for the ATF in West Virginia, has been charged in connection with the missing weapons and parts, the Journal newspaper in West Virginia reported earlier this month.

A search of Yates' vehicle yielded a pistol that had been registered in the government's archives of retired weapons in 2017, according to court documents. In an interview with ATF agents late last month, Yates also allegedly acknowledged transferring at least some of the stolen property to an unidentified person Maryland.

Yates' attorney, Nicholas J. Compton, declined to comment Wednesday.

ATF spokeswoman April Langwell described the theft as involving a "substantial" number of firearm parts and ammunition, including slides and barrels from retired ATF service weapons, but would not specify how many or where investigators believe they have ended up. Outside of the ATF, the materials are mostly unregulated and available for purchase online without a background check.

The search for the missing weapons was first reported by CBS News.

Langwell said the parts were likely sold to people who did not know they were stolen. She said the ATF first learned of the theft when a local law enforcement agency asked the ATF to trace a gun part it had recovered.

"ATF has made substantial progress in recovering the stolen property and is working around the clock to pursue all leads," the agency said in a statement.

Before the failures in the storefront operations, the ATF faced scrutiny for allowing more than 2,000 weapons to stream into Mexico as part of another botched undercover operation. One of the weapons was found at the scene of a fatal shooting of Border Patrol agent Brian Terry.

USA Today contributed to this report. Contact John Diedrich at (414) 224-2408 or jdiedrich@journalsentinel.com. Follow him on Twitter at @john_diedrich, Instagram at @john_diedrich, LinkedIn or Facebook.