Washington (CNN) President Donald Trump extended the deadline for the public release of the John F. Kennedy assassination files to 2021 on Thursday, prolonging the infamously drawn-out disclosure around the shooting.

Trump's move came on the deadline he imposed last year for the full release of the files -- barring national security and privacy concerns -- after the 25-year-in-the-making deadline imposed by the John F. Kennedy Assassination Records Collection Act came to pass last October.

"I agree with the Archivist's recommendation that the continued withholdings are necessary to protect against identifiable harm to national security, law enforcement, or foreign affairs that is of such gravity that it outweighs the public interest in immediate disclosure," Trump said in a presidential memorandum on Thursday.

The memorandum accompanied a release of about 19,000 documents by the National Archives in compliance with the records law and Trump's order last year.

Many of the documents released Thursday contain redactions, and they join the massive trove of Kennedy assassination records that already have been made public.

Kennedy's 1963 assassination prompted a whirlwind of questions from the public and researchers, plenty of conspiracy theories and reflexive secrecy from the government.

In 1992, President George H.W. Bush signed the JFK Assassination Records Collection Act, requiring the release of all records relevant to the assassination by the year 2017.

'Possible we'll just never see some'

Over the years, millions of documents have become public, offering researchers a major opportunity to pore over not only records related to the Kennedy assassination, but also a variety of other topics, from Dr. Martin Luther King Jr.'s life and slaying to pivotal moments in the Cold War.

Phil Shenon, an author and former New York Times reporter, told CNN it would take "weeks, months, years" to go through the existing documents, especially given duplicate documents and remaining redactions, in addition to the cloud of unreleased records.

"I think it's possible we'll just never see some of these documents," he said.

Shenon said the most credible evidence in his assessment lends credence to the notion that Lee Harvey Oswald was the lone gunman, but he said evidence within the documents publicly released since the 1992 law have underscored the possibility Oswald told others of his plans.

"We've gotten millions of pages of documents since this law was passed in 1992," Shenon said. "And a lot of those documents have pointed away from Oswald as a pure lone wolf. The documents show that he may have talked openly about killing Kennedy before the assassination and that he was in touch with people who may have wanted to see Kennedy dead."

Questions on future release

Last year's release was hotly anticipated, and ultimately the deadline came to pass with Trump allowing many documents to go public while granting government agencies a few months of breathing room -- a time period he extended by several years on Thursday.

CNN reported last October that Trump was "unhappy" with the incomplete release around the records deadline.

As they began to sift through the newly released documents, researchers focused on the continued delays in the full disclosure of the records.

Dr. Larry Sabato, a historian and professor of politics who studies the assassination, said in an email conversation with CNN that he is pessimistic the full records will ever come to light.

"Maybe scholars in 2063 will have more to work with," he said. "And that's not a flip statement."

Sabato pointed to fellow Kennedy researcher Gerald Posner, who identified on Twitter what he said were examples of redactions in Thursday's release of information that already had been made public.

In one example, Posner said a reference to "Mexican police" had been scrubbed possibly out of concern about harm to foreign relations, and in another, Posner said there were missing pages in a version of a document already released in full some 20 years ago.

Some evidence that redactions or w/holding on #JFKfiles in part due to concern re harm to "foreign relations" 1975 memo from a researcher lists "mexican police" surveillance camera at USSR Mexico City embassy. Today's release redacts what has been public https://t.co/TbCAZM7AAE pic.twitter.com/qIiN3ARrAR — Gerald Posner (@geraldposner) April 26, 2018