The federal government released a massive trove of documents late Thursday related to the era of President John F. Kennedy, the investigation into his 1963 assassination and the effort to learn about his killer, Lee Harvey Oswald, and Oswald's murderer, Jack Ruby.

An initial review of the thousands of pages of material — including FBI interviews, reports of CIA activity and other official but long secret records — surfaced household names of the era, from the actress Marilyn Monroe, to the mobster Sam Giancana and, over and over, John F. Kennedy, whose death in Dallas spelled an end to an age of American innocence.

Not all of the remaining Kennedy documents were released to the public. President Donald Trump deferred to a request from the FBI and CIA to withhold 300 documents for at least 180 days in the name of national security. Trump has called on any federal agency that wants to withhold the documents after that period to provide clear justifications related to specific documents.

The documents reviewed late Thursday did offer significant information related to the investigation into Kennedy's murder, the figures involved in it, and a Cold-War era defined by the government's fight against communism as well as its fear of the burgeoning civil rights movement.

The documents included:

CIA notes on a May 1964 conversation with Soviet leader Nikita Khrushchev, who said he didn't believe American security was so "inept" that Kennedy was killed without a conspiracy. Khrushchev believed the Dallas Police Department to be an "accessory" to the assassination. The CIA source "got the impression that Chairman Khrushchev had some dark thoughts about the American Right Wing being behind this conspiracy." When the source said that Oswald and Ruby both were "mad" and "acted on his own...Khrushchev said flatly that he did not believe this."

FILE - In this June 3, 1961, file photo, Soviet Premier Nikita Khrushchev and President John F. Kennedy talk in the residence of the U.S. Ambassador in a suburb of Vienna. (1961 File Photo / The Associated Press)

An FBI document relating threats against Oswald while he was in custody on the eve of his murder, saying the killing "after our warnings to the Dallas Police Department, was inexcusable." The same document, just days after the assassination, expresses concerns about the authorities' ability to "convince the public that Oswald is the real assassin."



A July 15, 1964 letter from the FBI to then-Attorney General Robert Kennedy advised him that a soon-to-be published book alleged that he was having an affair with actress Marilyn Monroe. But the book alleged Robert Kennedy was working with communists "behind the scenes" to have her killed, then covered up as a suicide, the letter said. The book also said Kennedy was at her apartment the night she died. Agents wrote that allegation was "branded as false as the Attorney General was actually in San Francisco with his wife at the time Marilyn Monroe committed suicide."



One FBI memo dated Nov. 23, 1963, recounted Oswald's visit to the Russian Embassy in Mexico City, where he tried speaking Russian, but he spoke "terrible hardly recognizable Russian." Soviet officials asked him multiple times to switch to English but he refused.



In this Nov. 23, 1963, file photo, surrounded by detectives, Lee Harvey Oswald talks to the media as he is led down a corridor of the Dallas police station. (AP Photo) (AP)

A large number of documents detailed the FBI's efforts to learn more about Jack Ruby and specifically his "hoodlum" background in Chicago and Dallas.



A CIA transcript dated July 26, 1965 gave a complete translation of an interrogation of Lt. Col. Yuri Ivanovich Nosenko, a KGB operative who defected to the U.S. in 1964. He claimed to have important information proving that Russia had no involvement in the Kennedy assassination. "I am ready to answer all questions," Nosenko said early in the interrogation.



A document, dated May 30, 1975, pertained to the 1975 Rockefeller Commission investigating CIA involvement of plans to assassinate foreign leaders. The document states that with respect to Fidel Castro, the "investigation is sufficiently complete to show that plans were undertaken by the CIA."



A note on a 1963 document said that a CIA source reported Fidel Castro becoming "extremely concerned with persistence of investigation into President Kennedy's murder and with possible disclosures that could result."



A November 27, 1963 urgent memo to FBI Director J. Edgar Hoover and the top special agent in Dallas told of an unfounded report from a California woman, Lillian Fisher, stating she expected Chief Justice Earl Warren to be assassinated at Kennedy's funeral. The statement "was based on pure conjecture," the memo stated.



After decades of waiting, researchers and conspiracy theorists had to wait a little longer Thursday — refreshing their web browsers for hours — during a delay in the release of the JFK files that lasted until after 5 p.m.

A Nov. 24, 1963 correspondence from the FBI detailing known threats on Oswald's life prior to his slaying.

President Donald Trump said he would release 2,800 of the documents Thursday, but bowed to national security concerns by the FBI and the CIA to allow another six months to review the remaining 300 records. Some of those may be redacted or never released, a Trump aide said.

"The American public expects and deserves its government to provide as much access as possible to the President John F. Kennedy assassination records so that the people may finally be fully informed about all aspects of this pivotal event," reads a memorandum Trump signed Thursday, which aides characterized as a temporary certification.

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"Therefore, I am ordering today that the veil finally be lifted," Trump wrote.

"I think what happened today was unfortunate," said Gerald Posner, author of Case Closed: Lee Harvey Oswald and the Assassination of JFK. a finalist for the Pulitzer Prize. "The expectations were built up, not just as a result of the president's tweets, but because 25 years ago Congress had given a deadline of Oct. 26, 2017."

The delay, he said, "adds to the perception that the average person has that the government has something to hide."

CIA notes on a May 1964 conversation with Soviet leader Nikita Khrushchev.

Dale K. Myers, author of With Malice: Lee Harvey Oswald and the Murder of Officer J.D. Tippit, said many of the documents released Thursday were withheld earlier because of privacy issues.

"My sources at the National Archives have been telling us for years that there's no smoking gun in any of this stuff," Myers said from his home in Michigan. "These are the pages that will mention who the informants were. So, we know the stories they told. We just don't know who told them. That's what's being released."

Staff Writer Michael Granberry contributed to this report.