The former U.S. military analyst, who was once dubbed "the most dangerous man in America" by top U.S. official Henry Kissinger, set off a major freedom-of-the-press battle in the 1970s.
He had worked at influential policy think tank the RAND corporation at the height of the Vietnam War. That's where the Pentagon sent two copies of a top secret report, covering U.S. involvement in Vietnam from 1945 to 1967.
The top secret study said U.S. officials had concluded the ongoing Vietnam War couldn't be won, and President John. F. Kennedy approved a coup to overthrow the South Vietnamese leader. It also said President Lyndon Johnson had plans to expand an aerial bombing campaign, despite saying he wouldn't whilst running for office. The papers also revealed secret U.S. bombing in Cambodia and Laos, and that casualty figures were higher than reported.
Ellsberg snuck the papers out of the RAND office, copied it, and eventually shared it with the media.
It led to a high-profile First Amendment fight between the New York Times and the Nixon administration, which invoked the Espionage Act to block the publication of the papers. The Supreme Court ruled in favor of freedom of the press.
Ellsberg and a RAND colleague were eventually charged with espionage, theft and conspiracy. But the judge threw out the case due to government misconduct, after it emerged the White House ordered a burglary at Ellsberg's psychiatrist's office, to search for evidence.
In his later years, Ellsberg became a writer and lecturer, campaigning for government transparency.
Ellsberg's family said he passed away at his home in Kensington, California, on Friday. He had been diagnosed with pancreatic cancer in February.
The once-top-secret papers that Ellsberg had leaked, can now be read online on the National Archives website.
