US Secretary of State Marco Rubio said a foreign actor was involved in an incident near RAF Fairford in western England, where five British men were arrested on suspected explosives and terrorism offences.
The airbase is used by US forces for operations against Iran, and British counterterrorism police are investigating whether individuals acting for a foreign state were involved.
The five suspects, all London residents in their early 20s, were released on bail on Monday as the investigation continued.
Police declared a major incident after the arrests early Sunday, while the British government held a national emergency meeting.
Authorities were alerted by a local farmer who found three large white vans parked near the base and reported seeing hooded and masked men running away.
A source familiar with the situation told Reuters that the vans did not contain viable explosives and that the police response was not intelligence-led.
Security at RAF Fairford had nevertheless been increased before Sunday, according to local residents and the Reuters source.
About 85 households in a nearby village were evacuated before residents were allowed to return, while roadblocks and a cordon remained around the base on Tuesday morning.
Rubio described the incident as a “very serious situation” and said it clearly involved a foreign actor, but provided no evidence or further details.
US President Donald Trump said the incident “might be” connected to Iran and criticized the decision to release the suspects on bail.
Iran denied involvement, with its embassy in London rejecting speculation linking Tehran to the incident.
Iran’s Islamic Revolutionary Guard Corps had said in July that RAF Fairford was a legitimate target.
Tehran has also previously warned Britain against allowing its bases to support US operations against Iran.
The UK gave Washington permission in March to use Fairford for specific defensive operations into Iran, and US bombers and other aircraft have since operated from the base.
British police have stressed that the investigation remains active despite the suspects’ release.