The National Archives unsealed thousands of pages from the Kennedy files on Friday. And while assassinations buffs weren't likely to find any major revelations — no proof of a second gunman, a Cuban plot, or evidence the killer could have been stopped — they'll have plenty to chew on.

The 3,539 records include FBI and CIA reports on Soviet spies, the assassination of the Rev. Martin Luther King Jr., and Lee Harvey Oswald's trip to Mexico City a few weeks before he murdered President John F. Kennedy in Dallas on Nov. 22, 1963.

This batch likely will be the last released pending a final review of records. Many remain sealed at the request of the FBI, CIA and other agencies that pressed for more time ahead of a deadline set a quarter century earlier.

For decades, debate has raged not only over whether Oswald acted alone but whether the FBI and CIA could have stopped him. The latest documents provide fresh proof that he was in their sights: a 1975 CIA memo marked "top secret" shows that Oswald was on a "watch list" of people whose mail would be intercepted from Nov. 9, 1959, to May 3, 1960, and again from Aug. 7, 1961, through May 28, 1962.

The same watch list included Francis Gary Powers, the U-2 pilot shot down on May 1, 1960. His mail was opened until two months after his release by the Soviets. CIA also opened the mail of Earl Browder, the head of the Communist Party of the United States, playwright Edward Albee, "Grapes of Wrath" novelist John Steinbeck, and a daughter of David Rockefeller, chairman of Chase Manhattan bank.

Another revelation comes from a July 1978 memo to an attorney on the staff of the House Select Committee on Assassinations: The FBI was unable to locate the original fingerprints lifted from the rifle found at the sniper's perch on the sixth floor of the Texas School Book Depository.

Dallas police turned those over a few days after the assassination and never got them back. Top FBI officials told House investigators that finding the prints would be a "mammoth research effort."

The head of the bureau's fingerprint section told House investigators that standard procedure would have required returning the original prints to Dallas police, but "this case was not routine, nor was it handled as such."

In 1992, Congress set Oct. 26, 2017, for the release of all remaining documents in the Kennedy collection. The National Archives released a batch in July, and five since then, including Friday's. In late October, President Donald Trump gave the agencies six more months to review any material that might damage national security.

"We don't pass judgment on the value of the information or draw any conclusions about the content. That's left for the American people - for journalists, researchers, historians and the like," said Jay Bosanko, chief operating officer at the National Archives.

Previous batches of documents have revealed the deep ties between U.S. and Mexican intelligence agencies, and the lengths the United States went to in attempts to undermine or assassinate Cuban dictator Fidel Castro.

A trove released Oct. 26 included a CIA report showing that Soviet leader Nikita Kruschev believed Dallas police had been an "accessory" to the assassination, because he found it implausible that presidential security was so "inept" for Kennedy to be killed without a conspiracy.

The documents have also revealed serious lapses in the surveillance of Oswald, a former Marine sharpshooter who had defected to the Soviet Union, then returned, then sought to go back. He was in Mexico City seeking visas to Cuba and then the Soviet Union.

Much of the material released Friday likely would not be subject to public disclosure under ordinary open records rules. Such materials provide insight into unrelated investigations, law enforcement techniques, foreign relations and intelligence gathering.

One 1990 FBI document, for example, relates a story from a source who claims U.S. Marines had unknowingly brought a Soviet agent into secure areas of the U.S. Embassy in Moscow "to engage in sexual intercourse." The agent then planted audio devices in the embassy, the source said, and removed the bugs during another sexual encounter.

Some of the documents also detail continued efforts to track the KKK and other white supremacist groups, especially in relation to the assassination of King. One set of FBI reports unsealed Friday details efforts to infiltrate the Ku Klux Klan in the late 1960s with help from an informant - a former Imperial officer - who "has been utilized extensively to cause dissension in Klan ranks."

The FBI scrambled for information on Dallas nightclub owner Jack Ruby in the days after he shot and killed Oswald.

Agents around the country contacted gambling sources for any insights, mostly coming up empty. But in Chicago, agents learned that Ruby had been close with Ross Prio, a "top hoodlum" in the city. And he was friends with gun shop owner Joe Scaramuzzo, who had sold three of the four guns used in a 1954 shooting at the U.S. House of Representatives by Puerto Rican nationalists.

Roughly a dozen archivists have been devoted to the JFK papers this year, coordinating with officials at the FBI, CIA, Defense Intelligence Agency and other federal agencies.

All documents subject to release have now been released in full or in redacted form, except for 86 records subject to further research by archivists and federal agencies. Records released only in redacted form will be reviewed in coming months by the agencies that generated them.

Most of the 5 million pages of Kennedy records were released in the 1990s, a bonanza for assassination buffs on a host of related topics, including FBI monitoring of anti-war groups, King, the Weathermen and others.

Congress created the five-member Assassination Records Review Board in 1992 as part of a law requiring the release of all Kennedy assassination documents within 25 years. The law authorizes the president — the one in office in 2017, that is — to block release if he deems it would harm U.S. intelligence, law enforcement, military or diplomacy interests.

Just under a third of the materials released Friday - an estimated 85,000 pages worth -- had been categorized as irrelevant to the JFK assassination itself, and withheld in full until now.

As with previous sets of records, this one provides insight into foreign intrigue and efforts to spy on adversaries.

There's a memo from the files of Kennedy's defense secretary, Robert McNamara, with a juicy geopolitical tidbit, courtesy of an informant close to the mother of Marxist leader Che Guevara. Che has recently returned from Cuba and told his mom that "Both he and Castro feel Khrushchev 'let them down' and has no further interest in spreading Communism in South America. Guevara added that he and Castro believe they and the Chinese are better Communists than Khrushchev."

The latest trove includes reports on Soviet and Cuban arms smuggling in the Caribbean, and complaints made to the Dallas FBI office about authors and documentary film makers contacting witnesses from the Grassy Knoll or the killing of Officer J.D. Tippett.

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