The CIA conspired with a Chicago gangster described as "the chieftain of the Cosa Nostra and the successor to Al Capone" in a bungled 1960 attempt to assassinate Fidel Castro, the leader of Cuba's communist revolution, according to classified documents published by the agency yesterday.

The disclosure is contained in a 702-page CIA dossier known as the "Family Jewels" compiled at the behest of then agency director James Schlesinger in 1973. According to a memo written at the time, the purpose of the dossier was to identify all current and past CIA activities that "conflict with the provisions of the National Security Act of 1947" - and were, in other words, illegal.

The dossier covers operations including domestic surveillance, kidnapping, infiltration of anti-war movements, and the bugging of leading journalists.

But its detailed information on assassination attempts against foreign leaders is likely to attract most attention.

The plot to kill Mr Castro, whom the US government at the time considered a threat to national security and a stooge of the Soviet Union, begins quietly and sinisterly in August 1960.

The documents released yesterday describe how a CIA officer, Richard Bissell, approached the CIA's Office of Security to establish whether it had "assets that may assist in a sensitive mission requiring gangster-type action. The mission target was Fidel Castro".

The dossier continues: "Because of its extreme sensitivity, only a small group was made privy to the project. The DCI (Director of Central Intelligence Allen Welsh Dulles) was briefed and gave his approval."

Following the meeting with the Office of Security, Bissell employed a go-between, Robert Maheu, and asked him to make contact with "gangster elements". Maheu subsequently reported an approach to Johnny Roselli in Las Vegas. Roselli is described as "a high-ranking member of the 'syndicate' (who) controlled all the ice-making machines on the (Las Vegas) Strip and (who) undoubtedly had connections leading into the Cuban gambling interests".

The CIA is careful to cover its tracks. According to the dossier, Maheu told Roselli that he (Maheu) has been retained by international businesses suffering "heavy financial losses in Cuba as a result of Castro's action. They were convinced that Castro's removal was the answer to their problem and were willing to pay the price of $150,000 (£75,000) for its successful accomplishment".

Roselli was also told that the US government was not, and must not become aware of the operation.

Roselli in turn led the CIA to a friend, known as Sam Gold. In September 1960, Maheu was introduced to Gold and his associate, known as Joe. In a development that appears to underscore the amateurishness of the whole operation, Maheu subsequently accidentally spotted photographs of "Sam and Joe" in Parade magazine.

Gold was in fact Momo Salvatore Giancana, "the chieftain of Cosa Nostra (the mafia) and the successor to Al Capone". Joe was actually Santos Trafficante, Cosa Nostra boss of Cuban operations.

At a meeting at the Fontainebleau Hotel in Miami Beach, Gold/Giancana suggested that rather than try to shoot or blow up Mr Castro, "some type of potent pill that could be placed in Castro's food or drink would be much more effective".

He said a corrupt Cuban official, named as Juan Orta, who was in debt to the syndicate and had access to the Cuban leader, would carry out the poisoning. The CIA subsequently obtained and supplied "six pills of high lethal content" to Orta but after several weeks of abortive attempts, Orta demanded "out" of the operation.

Another disaffected Cuban was recruited to do the job, but he demanded money up front. In the event, the dossier relates, "the project was cancelled shortly after the Bay of Pigs episode" (in April, 1961).

Yesterday's document release under the Freedom of Information Act also reveals details of CIA bugging and surveillance operations and the handling of a Soviet defector and KGB agent, Yuri Ivanovich Nosenko, in 1965-67. Also made public are 147 pages of documents relating to CIA assessments of the Soviet and Chinese cold war leaderships.

"The CIA fully understands it has an obligation to protect the nation's secrets, but it also has a responsibility to be as open as possible," CIA director Michael Hayden said yesterday. "The declassification of historical documents is an important part of that effort."

The documents are available at: www.foia.cia.gov/