LAS VEGAS—Robert A. Maheu, a former Howard Hughes confidant and CIA operative once involved in a failed plot to poison Fidel Castro, has died in a Las Vegas hospital. He was 90.

Maheu died Monday evening of congestive heart failure at Desert Springs Hospital, according to his son, Peter Maheu.

Maheu was the public face of Hughes’ massive corporate empire in the 1960s, a period in which the troubled aviator and one-time Hollywood playboy was increasingly reclusive and dogged by phobias. Hughes spent the later part of the decade holed up in his Las Vegas hotel suite, directing Maheu and his casino and development interests via memo.

Maheu said he occasionally protected the billionaire from himself.

“There were some memos where references pertaining to absolute power were sent to me. To protect him from himself, I took no action on those memos,” Maheu told The Associated Press in a 1986 interview. “To buy a particular president or to buy someone into the White House—no action was taken in that direction.”

Maheu was an expert in discretion.

He worked for the FBI in the early 1950s and later as a private investigator who counted the CIA among his clients.

Peter Maheu described his father as devout, energetic and loyal.

He returned to investigative work after he was abruptly fired by Hughes’ company shortly after its chief mysteriously slipped out Las Vegas in 1970. Maheu sued for breach of contract and won a $2 million judgment.

“He always had a respect for Howard Hughes,” Peter Maheu said. “But he felt that he was betrayed by the people around him.”