Four decades ago, a young defence analyst leaked a top-secret study packed with damaging revelations about U.S. conduct of the Vietnam War. On Monday, that study, dubbed the Pentagon Papers, finally came out in complete form.

The documents show that almost from the opening lines, it was apparent that the authors knew they had produced a hornet's nest.

In his Jan. 15, 1969, confidential memorandum introducing the report to the defence chief, the chairman of the task force that produced the study hinted at the explosive nature of the contents.

"Writing history, especially where it blends into current events, especially where that current event is Vietnam, is a treacherous exercise," Leslie H. Gelb wrote.

Asked by Defence Secretary Robert S. McNamara to do an "encyclopedic and objective" study of U.S. involvement in Vietnam from Second World War to 1967, the team of three dozen analysts pored over a trove of Pentagon, CIA and State Department documents with "ant-like diligence," he wrote.

The National Archives released the Pentagon Papers in full Monday and put them online, long after most of the secrets spilled.

40th anniversary

The release was timed 40 years to the day after The New York Times published the first in its series of stories about the findings, on June 13, 1971, prompting President Richard Nixon to try to suppress publication and crush anyone in government who dared to spill confidences.

Prepared near the end of Johnson's term by Defence Department and private analysts, the report was leaked primarily by one of them, Daniel Ellsberg, in a brash act of defiance that stands as one of the most dramatic episodes of whistleblowing in U.S. history.

As scholars pore over the 47-volume report, Ellsberg said the chance of them finding great new revelations is dim. Most of it has come out in congressional forums and by other means, and Ellsberg plucked out the best when he painstakingly photocopied pages that he spirited from a safe night after night, and returned in the mornings.

He told The Associated Press the value in Monday's release was in having the entire study finally brought together and put online, giving today's generations ready access to it.

Nixon's attempt to avenge the Pentagon Papers leak failed. First the Supreme Court backed the Times, The Washington Post and others in the press and allowed them to continue publishing stories on the study in a landmark case for the First Amendment of the U.S. Constitution that, among other things, guarantees freedom of the press.

Then the government's espionage and conspiracy prosecution of Ellsberg and his colleague Anthony J. Russo Jr. fell apart, a mistrial declared because of government misconduct.

The judge threw out the case after agents of the White House broke into the office of Ellsberg's psychiatrist to steal records in hopes of discrediting him, and after it surfaced that Ellsberg's phone had been tapped illegally.

The declassified report includes 2,384 pages missing from what was regarded as the most complete version of the Pentagon Papers, published in 1971 by Democratic Sen. Mike Gravel of Alaska. But some of the material absent from that version appeared — with redactions — in a report of the House armed services committee, also in 1971.

In addition, at the time, Ellsberg did not disclose a section on peace negotiations with Hanoi, in fear of complicating the talks, but that part was declassified separately years later.

The 40th anniversary provided a motivation for government archivists to declassify the records.

"If you read anything on the Pentagon Papers, the last line is always, 'To date, the papers have yet to be declassified by the Department of Defence,"' said A.J. Daverede, director of the production division at the National Declassification Center. "It's about time that we put that to rest."

The centre, part of the National Archives, was established by a 2009 executive order from President Barack Obama, with a mission to speed the declassification of government records.

Obama's administration has pursued cases against five government leakers under espionage statutes, more than any of his recent predecessors.

Most prominent among the cases is that of Pte. 1st Class Bradley E. Manning, an intelligence analyst accused of passing hundreds of thousands of military and State Department documents to WikiLeaks.

The administration says it provides avenues for whistleblowers to report wrongdoing, even in classified matters, but it cannot tolerate unilateral decisions to release information that jeopardizes national security.