WASHINGTON (Reuters) - The U.S. Secret Service said on Friday it is under investigation by the Department of Homeland Security over the loss of computer files on the Washington Metro system.

In 2008, a contract employee lost two computer tapes on the Metro while transporting them from one facility to another, Secret Service spokesman Edwin Donovan said. The investigation was first reported Friday morning by Fox News.

The Secret Service notified transit police and the Department of Homeland Security, but were unable to locate the tapes. The back up tapes were not marked or identified and were protected by many layers of security, Donovan said.

"It was a low risk for compromise," Donovan said.

Following the incident, the Secret Service put in place new procedures aimed at preventing a repeat.

"There has been no reported fraud associated with the loss of these tapes," Donovan said.

DHS did not immediately respond to a request for comment.

The Secret Service is under scrutiny after about a dozen employees were accused of misconduct for bringing women, some of them prostitutes, back to their hotel rooms ahead of a visit by President Barack Obama to Cartagena, Colombia.

(Reporting By Mark Felsenthal; editing by Todd Eastham)