 -- Documents released by the National Archives Thursday illustrated the broad swath of the investigation into the 1963 assassination of President John F. Kennedy, including memos about Communist sympathizers, anti-Fidel Castro activities and U.S. intelligence assets offering information about Cuba.

The documents related to the investigation into Kennedy's murder -- consisting of files from the CIA, the FBI, the Defense and State departments and other agencies -- were scheduled to be released 25 years after the passage of the President John F. Kennedy Assassination Records Collection Act of 1992. The law called for the records to be made available based on the approval of the president. Over 2,800 were made public Thursday, but some remained withheld due to national security concerns, according to a memo from President Donald Trump.

The collection, which spans from the early years of the Kennedy administration into the 1970s, includes discussions of investigative leads about assassin Lee Harvey Oswald’s travels, including a trip to Mexico before the assassination. Whom Oswald met with on that trip has been the subject of long-generated speculation.

One memo from then-FBI Director J. Edgar Hoover, dictated on Nov. 24, 1963, just hours after Jack Ruby shot Oswald, says the FBI had sent an agent to the hospital hoping for a confession from the alleged assassin before he died. After the attempt was unsuccessful, the memo illustrates Hoover's urgent desire to have "something issued so that we can convince the public that Oswald is the real assassin."

Another Hoover memo details information from a source within the USSR on the Soviet reaction to Kennedy's death. The source says the news was met with "great shock and consternation and church bells were tolled in the memory of President Kennedy." The Soviets were shocked by the development and preferred Kennedy as the head of the U.S. government as they felt they had a "mutual understanding" with him.

The Soviet Communist Party believed the assassination was an "ultraright" act and in effect a "coup." The source also said the Soviets immediately began instructing their agents to gather information on the new president, Johnson.

A document memorializing information obtained by the CIA said, "Circumstances already developed here point to possibility that Oswald may have been Castro’s agent. Mexicans are also keenly aware of the possibility." A note in the margin makes clear that the source of that information is unknown, and the information "varies" from at least one other account.

Many of the documents contain raw intelligence information that is uncorroborated, but will surely fuel already widespread speculation about the plot. The total collection contains more than 5 million records, making any single document a miniscule portion of all the intelligence.

Trump issued a memo to the heads of executive departments certifying the declassification Thursday, but also noted that some expressed reservations and therefore ordered that federal agencies be given 180 days to re-review whether certain documents related to national security require continued redaction or withholding.

"Executive departments and agencies have proposed to me that certain information should continue to be redacted because of national security, law enforcement, and foreign affairs concerns," reads the memo from Trump. "I have no choice -- today -- but to accept those redactions rather than allow potentially irreversible harm to our nation's security. To further address these concerns, I am also ordering agencies to re-review each and every one of those redactions over the next 180 days."

The 2,891 records that were released were posted on the National Archives' website, with more expected to be made public following the continued review.

Trump said on Twitter Friday that the files are being "carefully released," but his hope to get "just about everything to [the] public."

The vast majority of records related to the assassination -- roughly 88 percent -- have been available since the late 1990s, with an additional 11 percent of the documents released, with redactions, since then.