The 34-year-old man who killed 12 people at the Washington Navy Yard on Monday worked for a Fort Lauderdale-based company, giving him access to the heavily-secured complex.
The FBI identified Aaron Alexis, a former Navy reservist, as the gunman.
Alexis did information technology work for The Experts, Inc., giving him a valid pass to the Washington Navy Yard.
"Mr. Alexis had legitimate access to the Navy Yard as a result of his work as a contractor," said Assistant FBI Director Val Parlave.
The Experts, Inc. issued a statement Wednesday, saying the company "enlisted a service to perform two background checks and we confirmed twice through the Department of Defense his secret government clearance."
Company officials said its latest background check only found a traffic violation for Alexis, adding that they were working with the FBI and other agencies during the investigation.