British police said on Monday they were investigating whether individuals acting for a foreign state were involved in an apparent plot to attack an airbase used by the United States but said five suspects they had arrested would be released on bail.

The five men, all British nationals from London in their early 20s, were detained early on Sunday after police were alerted by a local farmer about three white vans driving towards RAF Fairford, a British base that houses U.S. bombers that fly missions against Iran.

Iran’s Revolutionary Guards said in July the base was a legitimate target.