Partial records from long-secret files relating to the 1963 assassination of President John F Kennedy have been released – capturing the frantic days following the killing as officials madly chased after tips, juggled rumours and sifted through leads worldwide.

The final cache of official secret papers had been expected to be released on Thursday, but US President Donald Trump delayed the publication of hundreds of records.

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In the end, 2,800 documents were released, while others were kept secret because of national security concerns. The unpublished files will be further reviewed for the next six months.

Among the records to be published on Thursday were cables, notes and reports stamped “Secret” that reveal the suspicions of the era around Cubans and Communists.

They cast a wide net over varied activities of the Kennedy administration, such as its covert efforts to upend Fidel Castro’s government in Cuba.

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In the chaotic aftermath of the assassination, followed two days later by the murder of the shooter, Lee Harvey Oswald, while in police custody, FBI director J Edgar Hoover vented his frustration in a formerly secret report found in the files. It opened: “There is nothing further on the Oswald case except that he is dead.”

Records from the National ArchivesRecords from the National Archives (Jon Elswick/AP)

But, reflecting on Oswald less than an hour after he died, Hoover already sensed theories would form about a conspiracy broader than the lone assassin.

“The thing I am concerned about, and so is (deputy attorney general) Mr Katzenbach, is having something issued so we can convince the public that Oswald is the real assassin,” he said.

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He also reported: “Last night we received a call from our Dallas office from a man talking in a calm voice and saying he was a member of a committee organised to kill Oswald.”

Hoover said he relayed that warning to Dallas police and was assured Oswald would be sufficiently protected. Oswald was shot dead the next day by Jack Ruby.

A magazine seller in Dealey Plaza, Dallas, the site of the assassinationA magazine seller in Dealey Plaza, Dallas, the site of the assassination (LM Otero/AP)

A document from 1975 contains a partial deposition by Richard Helms, a deputy CIA director under Kennedy who later became CIA chief, to the Rockefeller Commission, which was studying unauthorised CIA activities in domestic affairs.

Commission lawyers appeared to be probing for information on what foreign leaders might have been the subject of assassination attempts by or on behalf of the CIA.

A lawyer asks Helms: “Is there any information involved with the assassination of President Kennedy which in any way shows that Lee Harvey Oswald was in some way a CIA agent or agent” – here the document ends, short of his answer.

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Also among the files is a more than 400-page document that appeared to describe people being monitored as potential threats to Kennedy and his successor, Lyndon B Johnson.

The full record will still be kept from the public for at least six months – and longer if agencies make a persuasive enough case for continued secrecy.

The collection includes more than 3,100 records – comprising hundreds of thousands of pages – that have never been seen by the public. About 30,000 documents were released previously, with redactions.

The files are not expected to give a definitive answer to a question that still lingers for some: Whether anyone other than Oswald was involved in the assassination.

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The Warren Commission in 1964 concluded that Oswald had been the lone gunman, and another congressional probe in 1979 found no evidence to support the theory that the CIA had been involved.

But other interpretations, some more creative than others, have persisted.