WASHINGTON (Reuters) - A U.S. Secret Service agent assigned to President Barack Obama's protective detail was found dead on Saturday in an apparent suicide, a law enforcement source said on Thursday.

Special agent Rafael Prieto was under investigation for having an unreported romantic relationship with a foreign national, the source said.

The agent's death is being investigated by the District of Columbia police, said Max Milien, a spokesman for the Secret Service.

"Rafael Prieto has a distinguished 20-year career with the Secret Service that was marked by accomplishment, dedication, friendships and the Secret Service is mourning a valued colleague," said Milien.

Prieto was not under investigation for compromising sensitive or classified information, but because he was involved in an administrative proceeding to see if he was in violation of Secret Service rules, the source said.

Gwendolyn Crump, a spokeswoman for the D.C. Metropolitan Police Department, would not provide details about an ongoing investigation.

The Secret Service, which prides itself in unobtrusively protecting the president and vice president, has found itself under unaccustomed public scrutiny in recent months.

Secret Service employees were accused of bringing women, some of them prostitutes, back to their hotel rooms in Colombia in April during a trip to prepare for a visit by the president. The agency said later that nine personnel were found to have been involved in serious misconduct.

A government investigation concluded their actions did not compromise the president's safety, but the event was an embarrassment for the Secret Service and its director issued a stricter code of conduct afterward.

A Secret Service officer was arrested in Miami last month the day after a visit by Obama after having been found passed out on a street corner.

Officers are uniformed members of the agency, and perform support services, in contrast to plain-clothed agents who directly guard the president and vice president.