The U.S. Justice Department has accused United States Investigations Services (USIS), the largest private provider of security checks for the government, of bilking millions of dollars through improper background verifications, Reuters reports.

USIS has had a contract with the U.S. government since 1996 to vet individuals seeking employment with federal agencies, and the checks include investigative fieldwork on each application.

The Department of Justice said in a court filing Wednesday that between March 2008 and September 2012, USIS filed at least 665,000 flawed background checks – about 40 percent of the total submissions.

DOJ says that USIS received millions of dollars that it otherwise would not have received had the government been aware that the background investigations had not gone through the quality review process that the contract requires, Reuters reports.