WASHINGTON — President Trump ordered the long-awaited release on Thursday of more than 2,800 documents related to the assassination of President John F. Kennedy, but bowed to pressure from the C.I.A. and F.B.I. by withholding thousands of additional papers pending six more months of review.
While incomplete, the documents represented a treasure trove for investigators, historians and conspiracy theorists who have spent half a century searching for clues to what really happened in Dallas on that fateful day in 1963. They included tantalizing talk of mobsters and Cubans and spies, Kremlin suspicions that Lyndon B. Johnson was behind the killing and fear among authorities that the public would not accept the official version of events.
Paging through the documents online on Thursday night was a little like exploring a box of random documents found in an attic. There were fuzzy images of C.I.A. surveillance photos from the early 1960s; a log from December 1963 of visitors, including a C.I.A. officer, coming and going from President Johnson’s ranch in Texas; and reports that Lee Harvey Oswald obtained ammunition from a right-wing militia group.
Some of the documents convey some of the drama and chaos of the days immediately following the murder of the president. Among them is a memo apparently dictated by J. Edgar Hoover, the F.B.I. director, on Nov. 24, 1963, shortly after Jack Ruby shot and killed Oswald as he was being moved from police headquarters to a local jail.
“There is nothing further on the Oswald case except that he is dead,” the memo begins laconically, before reciting the day’s events.
Mr. Trump, who has indulged in his own wild speculation about the sensational killing, had expressed eagerness to finally open the last of the government files, only to run into a last-minute campaign by intelligence agencies to redact certain documents. Grudgingly, he gave the agencies until April 26 to go through the remaining papers again and make their case.
“I am ordering today that the veil finally be lifted,” Mr. Trump said in a memo to the agencies. Given their objections, he said, “I have no choice — today — but to accept those redactions rather than allow potentially irreversible harm to our nation’s security.” But he ordered the agencies to “be extremely circumspect,” noting that the rationale for secrecy has only “grown weaker with the passage of time.”
For conspiracy theorists, the Kennedy assassination has been the holy grail, one that has produced an endless string of books, reports, lectures, articles, websites, documentaries and big-screen Hollywood movies. It was the first murder of an American president in the television age, touching off a wave of global grief for a charismatic young leader while also spawning a cottage industry of skeptical questioning of the official version of events.
Every government authority that has examined the investigation of his death, from the Warren Commission to congressional investigators, concluded that Kennedy was killed by Lee Harvey Oswald, who fired three shots with a mail-order rifle from the sixth floor of the Texas School Book Depository when the presidential motorcade passed by on Nov. 22, 1963. But that has never satisfied the doubters, and polls have consistently shown that most Americans still believe that someone other than Oswald must have been involved.
While the Warren Commission concluded that Oswald acted alone, the House Select Committee on Assassinations said in a 1979 report that Kennedy “was probably assassinated as a result of a conspiracy” but did not identify who those conspirators might have been. It ruled out the Soviet and Cuban governments, organized Cubans opposing Fidel Castro, the Mafia, the F.B.I., C.I.A. and the Secret Service, although it said it could not preclude that individuals affiliated with some of those groups might have been involved.
Among the doubters have been Mr. Trump, who last year alleged that the father of his Republican rival, Senator Ted Cruz of Texas, was somehow involved in the assassination. The president’s longtime friend and adviser, Roger J. Stone Jr., authored a book accusing Johnson of being responsible for the shooting that elevated him to the presidency.
As it happened, Mr. Trump’s deferral to the C.I.A. and F.B.I. invariably will lead to suspicions that the government is still protecting sensational secrets about the case. Administration officials said there was no cover-up, just an effort to avoid compromising national security, law enforcement or intelligence gathering methods.
The C.I.A., which has borne the brunt of suspicions from amateur assassination theorists for years, went out of its way on Thursday to try to dispel concerns that it was hiding important evidence.
The agency issued a statement noting that the vast majority of assassination-related records have been released, and that redactions were intended “to protect information in the collection whose disclosure would harm national security — including the names of C.I.A. assets and current and former C.I.A. officers, as well as specific intelligence methods and partnerships that remain viable to protecting the nation today.”
The release of the documents owes as much to the moviemaker Oliver Stone as anyone else. After his 1991 conspiracy theory movie, “J.F.K.,” stoked renewed interest, Congress passed the President John F. Kennedy Assassination Records Collection Act, which was signed into law by President George Bush on Oct. 26, 1992. The act mandated that all assassination records be released no later than 25 years from that date, which was Thursday, unless the president authorized further withholding for national security reasons.
In the years since the law was passed, the National Archives and Records Administration has released 88 percent of those documents in full and an additional 11 percent with portions redacted. Until Thursday, just 1 percent had been withheld in full.
Of the 2,891 documents released on Thursday, just 53 had never been disclosed by the archives; the rest had been made public with redactions.
The papers range widely and while many are not directly related to the assassination, others add context. One recounted the reaction of the Soviet Union to the killing, reporting that some in Moscow assumed it was a “coup” by the “ultraright” that would be blamed on the Soviet Union. An unnamed informant told American spies that the K.G.B. had proof that “President Johnson was responsible for the assassination.”
An F.B.I. cable from April 1964 reconstructed Oswald’s bus trip to Mexico weeks before the assassination, including the names of the people sitting around him and even what he was wearing: “a short-sleeved light colored sport shirt and no coat.”
In Hoover’s memo two days after the assassination, he expressed anxiety that Oswald’s killing would generate doubts among Americans. “The thing I am concerned about,” he wrote, “is having something issued so that we can convince the public that Oswald is the real assassin.” The F.B.I. director also fretted that discoveries that Oswald contacted the Cuban embassy in Mexico City and sent a letter to the Soviet Embassy in Washington could “complicate our foreign relations.”
He called the Oswald killing “inexcusable” in light of “our warnings to the Dallas Police Department” and hinted at Ruby’s mob connections, which would soon spawn an industry of research and speculation. “We have no information on Ruby that is firm, although there are some rumors of underworld activity in Chicago,” Hoover wrote.
The documents will not end the debate or speculation — and a few may add to the questions. In a 1975 deposition, for example, Richard Helms, the former C.I.A. director, was asked: “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 an agen…”
But the document ends there, and Mr. Helms’s answer is missing.