British police were questioning five men on Monday on suspicion of explosives and terrorism offenses after they were detained near an air base that has been used by the US to strike Iran.

Axar.az reports, citing Reuters, officers detained the five men in the early hours of Sunday after they received a tip-off that three large white vans had been seen heading to the military airfield of RAF Fairford in the west of England.

US President Donald Trump said the suspects had been looking to do “big damage” and had been under investigation by British and US authorities. However, a local farmer said she had tipped off police after seeing a group of “hooded and masked men” running into the fields.

British counterterrorism detectives have not yet publicly stated who they assessed was involved but a source familiar with the investigation said an Iranian-linked motive was considered the most likely, while a Russian sabotage operation or an extremist plot were also being looked at as possibilities.

Defense minister Wes Streeting told BBC News on Monday that he would not speculate about motives of a live investigation.

But he added on the general Iranian threat to the country: “We know that Iran and their proxies wish to do us harm. We know that they hate our democracy, and we know that we regularly have to thwart Iranian attempts to threaten either our interests or indeed our allies.”