Donald Trump's blowing the lid off secret files that may reveal the truth behind some of the most consequential political assassinations in American history.

The POTUS signed an order Thursday ... ordering the FBI and other agencies to release ALL documents relating to the assassinations of John F. Kennedy, his brother Robert, and Martin Luther King Jr. ... and he wants full disclosure STAT.

Trump signed the order in the Oval Office ... telling the crowd people have been waiting for this moment for decades.

Trump also told an aide to give the pen used to sign the document to Robert F. Kennedy Jr. -- the son of RFK and his pick for Human and Health Services Secretary.

Trump says there are no national security concerns that will be compromised by the release of the documents. In the case of JFK, the word on the streets of D.C. was that a foreign government -- possibly Cuba -- was behind the assassination.

The plan for releasing the JFK files must be presented in 15 days. A plan for releasing the RFK and MLK documents must be made in 45 days.

Records related to President Kennedy's murder have been released slowly over the years ... and, President Trump promised during his first term that he would facilitate the release of the documents still classified.