They wrote that the documents relate to what they call a “mysterious chapter in the history of the assassination — a six-day trip that J.F.K. assassin Lee Harvey Oswald paid to Mexico City several weeks before the president’s murder, in which Oswald met with Cuban and Soviet spies and came under intensive surveillance by the C.I.A.’s Mexico City station. Previously released F.B.I. documents suggest that Oswald spoke openly in Mexico about his intention to kill Kennedy.”

With the Oct. 26 deadline to release the remaining documents fast approaching, Mr. Trump had been under increasing pressure from advocates of transparency not to hold back any of the documents from the public on the grounds of national security.

Senator Charles E. Grassley, Republican of Iowa, introduced a resolution in the Senate this month that urged Mr. Trump to make a “full public release of all remaining records,” saying that he should “reject any claims for the continued postponement of the full public release of those records.”

Conspiracy theorists have long clamored for what they hope will be evidence to prove that the government covered up the truth about the assassination. This week, Roger J. Stone, a friend of Mr. Trump’s, told Alex Jones, the radio host and conspiracy theorist, that Mr. Stone had directly urged the president to release all the documents.

“I had the opportunity to make the case directly to the president of the United States by phone as to why I believe it is essential that he release the balance of the currently redacted and classified J.F.K. assassination documents,” Mr. Stone said on Mr. Jones’s radio program.

Mr. Trump is no stranger to conspiracy theories, including those involving the Kennedy assassination. During the presidential campaign, he at one point alleged that the father of Senator Ted Cruz, a Republican primary rival from Texas, had been with Oswald shortly before Kennedy was killed.

“You know, his father was with Lee Harvey Oswald prior to Oswald’s being — you know, shot,” Mr. Trump told Fox News in an interview in May 2016, as he battled the Texas senator for the nomination. “I mean, the whole thing is ridiculous. What is this, right, prior to his being shot, and nobody brings it up. They don’t even talk about that. That was reported and nobody talks about it. But I think it’s horrible.”