Sen. Jon Tester on Friday sent a letter to the Justice Department asking it to investigate a federal agency for reportedly stashing away millions of dollars in a secret bank account to finance undercover investigations and pay informants – and buy luxury items.

The scheme allegedly began in 2006 when the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives launched a covert investigation into cigarette smuggling. The U.S. Justice Department never authorized the program and the secret fund wasn’t revealed until 2013.

While four years have since passed, Tester said the Justice Department has yet to take action against those were responsible for the cover up.

“This secret multi-million dollar ATF slush fund appears to be a gross abuse of taxpayer dollars,” Tester wrote in a letter to Michael Horowitz, inspector general of the U.S Justice Department. “These allegations raise serious questions about the oversight capability of one of our top law enforcement entities, and I believe enough information exists to warrant an investigation.”

Reports contend that the ATF purposefully ran a multi-million-dollar slush fund in order to skirt congressional and executive oversight. The reports also allege that ATF used the fund to buy luxury travel items, including vehicles, personal expenses and a suite at a NASCAR event.

“I am deeply troubled by reports that the ATF purposefully ran this slush fund in order to skirt congressional and administrative oversight,” Tester said. “What began as a tool for investigations into tobacco smuggling quickly morphed into a blank check for ATF agents.”