A newly released cache of FBI files has revealed a potential plot to assassinate Queen Elizabeth 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 "either 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.
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.
Another teenager fired six blanks at her during the Trooping the Colour parade in central London. (AFP)