British intelligence officers scoured hundreds of UFO sighting reports in the 1980s and 1990s looking for top-secret American stealth-plane projects, newly released files have shown.
At the end of the cold war, the Ministry of Defense was worried that America was developing mysterious planes but not telling its closest ally.
Staff at department DI55 of the Defense Intelligence Staff logged and investigated hundreds of sightings by pilots, air traffic controllers and private individuals who believed they had seen an alien presence.
"The MoD did not believe Britain was being visited by aliens, but these files show they were obsessed with the fear that our closest ally was developing spy planes and keeping them secret from us," said David Clarke, a lecturer at Sheffield Hallam University and UFO specialist at Britain's National Archives (which on Sunday released hundreds of official UFO files on its Web site).
In one incident logged in 1989, an oil-rig worker trained in aircraft observation saw a "perfect black triangle" escorted by two American fighters.
An official noted: "Attention is now focusing on this, despite a recent U.S. Air Force 'denial briefing.' This is bound to prompt parliamentary interest."
In 1990, the crews of six RAF Tornado jets flying over Germany reported seeing a UFO. Staff at RAF West Drayton, where many sightings were initially collated, noted that the pilots had reported the "aerial phenomenon" overtaking them, and said: "Possibly idented [identified] as a stealth aircraft."