Police in the United Kingdom said Thursday they had arrested a 25-year-old British Iranian dual national in London following a security incident at RAF Fairford air base on Sunday. The U.K. has allowed the U.S. to launch attacks on Iran from the air base since March. Authorities have not named the man, who’s being held on suspicion of preparation of terrorist acts — the same charge levied against five British nationals who were arrested outside RAF Fairford, then released on bail.

On Thursday, British Prime Minister Andy Burnham said his government had “strong indications” of Iranian involvement in the incident. But exactly what took place outside the air base remains unclear. A police search of vans being used by the men turned up no signs of explosives. However, the vans did contain empty barrels, large tanks, siphoning pumps and fuel, leading to widespread speculation that the men were stealing petrol rather than plotting an attack.