This was written in a book that a Para wrote. He witnessed it personally and his mate was personally threatened by MI5/6 after the war to keep his mouth shut. The Americans were supposedly executed, on order from London (and IIRC in collusion with Washington) in order to stop an international incident.
Or so he says...
I just read a blurb about The Feather Men which the movie Killer Elite was based on. Whenever I hear of conspiracies ending in "everyone was killed to protect the secret" my BS meter pegs off the scale.