Declassified US government documents show that a man suspected of involvement in the bombing of a Cuban passenger plane worked for the CIA.
Luis Posada Carriles was accused of plotting to kill Fidel Castro
Luis Posada Carriles, a Cuban-born Venezuelan and anti-Castro dissident, was an agent and informer.
The papers also reveal that an FBI informer "all but admitted" that Mr Posada was one of those behind the 1976 bombing that killed 73 people.
Mr Posada, who denies any involvement, is said to be seeking asylum in the US.
His lawyer says his client, thought to be in hiding in the Miami area, deserves US protection because of his long years of service to the country.
US officials say they have no evidence that Mr Posada is in the country, and add that they would deal with an asylum application from him as they would any other.
The documents, released by George Washington University's National Security Archive, show that Mr Posada, now in his 70s, was on the CIA payroll from the 1960s until mid-1976.
One FBI report quoted a confidential source as saying that Mr Posada was one of several people who met at least twice at a hotel in Caracas, allegedly to discuss bombing a Cubana airlines plane.
The report recommended that no action be taken on the information, as it would compromise its source.
Mr Posada was arrested in Venezuela after the bombing, but was not convicted before he escaped from prison.
The US documents show that he later went to central America, where he joined the covert US operation, led by Lt Col Oliver North, to rearm the anti-communist Contra guerrillas.
Mr Posada once boasted of being responsible for a series of bomb attacks on Havana tourist spots in the 1990s.
Five years ago, he was arrested in Panama and accused of plotting to kill President Fidel Castro during a summit there.
He was convicted of a lesser charge, but was later pardoned and freed by the outgoing Panamanian president - causing Cuba to break off diplomatic relations.
His alleged reappearance in Miami has provoked the wrath of the Cuban government, which accuses Washington of harbouring an alleged terrorist.
Venezuela - a close ally of Cuba - says it is planning to seek his extradition. If Mr Posada has applied for asylum, his case will present the Bush administration a dilemma, says the BBC's Paul Keller in Miami.
The US would have to reconcile its traditional sympathy for the politically influential Cuban exiles in Miami and its firm stand against suspected terrorists in the wake of the 11 September 2001 attacks, our correspondent says.