British Prime Minister Andy Burnham said Wednesday that authorities have strong indications that Iran was involved in the security incident near RAF Fairford, while stressing that the investigation remains ongoing.

Five men in their 20s were arrested Sunday near the U.S.-used military base in Gloucestershire after police responded to reports of suspicious vans and masked men in the area.

The suspects were initially arrested on suspicion of explosives offenses and later on suspicion of preparing an act of terrorism. Police found a quantity of gasoline in the vans, but no explosive devices, and all five were released on strict bail conditions while remaining under investigation.

Burnham said British authorities are working closely with U.S. officials as investigators examine possible foreign involvement.

"There are strong indications that Iran played a part in what happened over the weekend at RAF Fairford," Burnham told the BBC, adding that it remains a "complex and serious police investigation."

The Prime Minister said he could not disclose the evidence behind the assessment but indicated that authorities would provide more information as the investigation develops.

U.S. Secretary of State Marco Rubio had previously said the incident appeared to involve "the hand of a foreign actor," while President Donald Trump also suggested the suspects' actions could be connected to Iran.

Iran has denied involvement in the incident. RAF Fairford has been used by U.S. military aircraft for operations against Iran, making the suspected incident particularly significant as investigators examine whether the men were acting on behalf of a foreign state.