"There are these glitches in Oswald's biography in which we don't know what he is up to. One of those is Mexico City," said Mr Posner, whose 1993 book Case Closed debunks the many conspiracy theories surrounding Mr Kennedy's assassination.
Oswald visited the embassies to apply for visas that would let him return to the Soviet Union, where he lived from 1959 to 1962. The CIA closely monitored both embassies. Posner said the CIA may have video of Oswald's visits to them.
The documents may also shed light on ex-CIA officer and Watergate conspirator E. Howard Hunt's confession to two of his sons that he had advance knowledge of rogue CIA officers' plans to kill Mr Kennedy.
Many assassination experts believe Hunt's confession soon before he died was too vague to mean anything. "Let's see what the documents show," said Posner.
For decades, the National Archives has been gathering government assassination papers.
Eighty-eight per cent of the Archives' 5 million pages of JFK material are already public.
Another 11 per cent are partly public, with sensitive portions removed. Just one per cent of the records remain fully secret.
Posner said the archive opening will be "the last big gasp for JFK documentation."
The US government's conclusion that Oswald shot Mr Kennedy on his own in a bid to attain a bizarre kind of fame is a hard sell. A 2013 Gallup poll found 61 per cent of Americans believe Mr Kennedy's assassination was a conspiracy.
Legions of JFK conspiracy theorists offer a suspect list that includes rogue CIA agents, the FBI, the Mafia, pro-Castro Cubans, anti-Castro Cubans, Corsican mobsters, and Lyndon Johnson, Kennedy's vice president.
Mr Posner - whose book was a finalist for a Pulitzer Prize in history - doubts the new documents will reveal any secret plot.
"If American intelligence had evidence of it, it would have been out a long time ago," he said.
Government archivists also doubt that the secret documents contain any startling revelations.
"We assume that much of what will be released will be tangential" to the assassination, the National Archives website says.
Researchers are nonetheless eager to see the documents. "Things that didn't look very exciting to them back in 1997 or 1998 might have relevance today," said Mr Posner.
Revealing the secret details will help Americans grasp what happened in Dallas, said William Kelly, an assassination researcher from New Jersey. "It's going to give us the final pieces of the puzzle," Kelly said.
