The files show that an officer in San Francisco warned federal agents about a man who claimed he wanted to seek revenge for his daughter's death in Northern Ireland.

The threat came a month before the Queen's visit to California, and the FBI planned to close the Golden Gate Bridge as the Queen's yacht sailed under it.

The documents also reveal that the FBI remained vigilant to potential threats to the Queen during her visits to the US, including a pilot who flew a small plane over Battery Park with a sign that read "England, Get out of Ireland." The FBI warned of potential threats from the Irish Republican Army during the late Queen's visit to Kentucky in 1989.