A newly released cache of FBI files has revealed a potential plot to assassinate Queen Elizabeth II during her 1983 visit to California.
The possible threat followed a phone call made by "a man who claimed that his daughter had been killed in Northern Ireland by a rubber bullet", according to the document that also refers to a bar frequented by Irish Republican Army (IRA) sympathisers.
The queen and her husband Prince Philip visited the west coast of the United States in February and March 1983, and the trip passed off without incident.
Four years earlier in 1979, IRA paramilitaries opposed to British rule in Northern Ireland killed Louis Mountbatten, the last colonial governor of India and an uncle of Philip, in a bomb attack.
The file states that the man claimed he was going to attempt to harm the queen "by dropping some object off the Golden Gate Bridge onto the royal yacht Britannia when it sails underneath".
Alternatively he "would attempt to kill Queen Elizabeth when she visited Yosemite National Park", they added.
A separate file among the documents, dated 1989, pointed out that while the FBI was unaware of any specific threats against the queen, "the possibility of threats against the British monarchy is ever present from the Irish Republican Army".
The queen, who died last September aged 96, has previously been reported to have been the target of other assassination plots.
Six blanks fired
In 1970, suspected IRA sympathisers unsuccessfully attempted to derail her train west of Sydney, while in 1981 the IRA tried to bomb her on a visit to Shetland, off the northeast coast of Scotland.
In the same year, a mentally disturbed teenager fired a single shot towards the queen's car during a visit to New Zealand.
Christopher Lewis fired a single shot as she toured the South Island city of Dunedin.
The botched attempt was covered up by police at the time and only came to light in 2018 when New Zealand's Security Intelligence Service (SIS) spy agency released documents following a media request.
Also in 1981, another teenager fired six blanks at her during the monarch's Trooping the Colour birthday parade in central London.
The queen quickly calmed her startled horse and carried on while the teenager told soldiers who disarmed him he had "wanted to be famous".
The following year, in one of the most famous security breaches of her reign, Michael Fagan managed to get into the queen's bedroom and spent 10 minutes talking to her before she could raise the alarm.
The unemployed decorator had a few drinks and scaled the walls of Buckingham Palace, climbing up a drainpipe to enter the queen's London residence.
He wandered into her bedroom and reportedly sat on the end of the bed for a chat with the perturbed monarch before a palace staffer lured him away with the promise of a shot of whisky.