Queen Elizabeth II faced a potential assassination threat during a 1983 visit to the United States, newly released FBI documents show.
The Federal Bureau of Investigation has released a cache of files relating to the late Queen's travels to the US, following her death last year.
They show how the FBI, which helped secure the monarch during her visits, worried about IRA threats.
The assassination threat was made to a police officer in San Francisco.
According to the file, an officer who frequented an Irish pub in San Francisco warned federal agents about a call from a man he had met at the venue.
The officer said the man told him he was seeking revenge for his daughter who "had been killed in Northern Ireland by a rubber bullet".
The threat came on 4 February 1983 - about a month ahead of Queen Elizabeth II and her husband Prince Philip's visit to California.
"He was going to attempt to harm Queen Elizabeth and would do this either by dropping some object off the Golden Gate Bridge onto the Royal Yacht Britannia when it sails underneath, or would attempt to kill Queen Elizabeth when she visited Yosemite National Park," the document says.
In response to the threat, the Secret Service had planned to "close the walkways on the Golden Gate Bridge as the yacht nears". It is unclear what measures were taken at Yosemite, but the visit went ahead. No details of arrests were published by the FBI.
The 102-page cache was uploaded to the Vault, the FBI's information website, on Monday, following a Freedom of Information Act request submitted by US media outlets.
Many of the late Queen's state visits to the US, including the 1983 visit to the West Coast, came during heightened tensions amid the Troubles in Northern Ireland.
In 1976, the late Queen was in New York City for America's Bicentennial celebrations.
The documents reveal how a summons was issued to a pilot for flying a small plane over Battery Park with a sign that read "England, Get out of Ireland."
The files show how the FBI remained vigilant to what it considered to be the real potential of threats to the late Queen.
Her second cousin Lord Mountbatten was killed in an IRA bombing off the coast of County Sligo, Republic of Ireland, in 1979.
Ahead of a personal visit by the late Queen to Kentucky in 1989, an internal FBI memo read "the possibility of threats against the British Monarchy is ever-present from the Irish Republican Army (IRA)".
It continued that "Boston and New York are requested to remain alert for any threats against Queen Elizabeth II on the part of IRA members and immediately furnish same to Louisville," in Kentucky.
The late Queen, who owned racehorses, is known to have visited Kentucky several times during her life to enjoy the state's equestrian highlights, including the Kentucky Derby.
On a state visit in 1991, the late Queen was scheduled to see a Baltimore Orioles baseball game with President George H Bush.
The FBI warned the Secret Service that "Irish groups" were planning protests at the stadium and "an Irish group had reserved a large block of grandstand tickets" to the game.
The bureau told NBC News there might be "additional records" that exist besides the ones released this week, but it did not set out a timetable for their publication.