U.S. Secretary of State Marco Rubio said on Monday that a “foreign actor” was involved in a suspected bomb plot at RAF Fairford, an English airbase used by American bombers striking Iran, hours after British police released five arrested suspects on bail.

The five men, British nationals from London aged between 23 and 25, were arrested in the early hours of Sunday near the base in Gloucestershire after a farmer alerted police to three white vans blocking a road in the village of Whelford, according to a statement from Counter Terrorism Policing. Officers found no explosive devices in the vehicles after an examination with military and forensic support, but recovered “a quantity of petrol,” the force said.

Assistant Commissioner Laurence Taylor, head of national counterterrorism policing, said the men had been arrested on suspicion of offences under the Explosives Act and later under Section 5 of the Terrorism Act, on suspicion of preparing a terrorist act. Announcing their release on police bail, he said the men “absolutely remain under investigation” and are subject to “strict conditions” on their movement and contacts.

“I can confirm that following extensive interviews, and inquiries, the five men who were arrested are being released on police bail this afternoon.”

Taylor said investigators were keeping an “open mind” and looking at the case “from every possible angle,” including the possibility that the men were “proxies, or individuals either knowingly or unknowingly, working on behalf of a foreign state.” He did not name a country.

Rubio went further, telling Fox News’ Hannity that the episode “clearly involves the hands of a foreign actor,” though he said he could not “get into great detail about that yet” and promised “a lot more news” in the coming days, according to Al Jazeera.

President Donald Trump told reporters on Monday he was “surprised” British police had freed the suspects, adding, “I wouldn’t have done that.” A day earlier he said the U.K. and U.S. had been investigating the men “for a long time” and that they “were looking to do big damage to our fort.”

Iran’s embassy in London rejected any link to the incident, saying it “categorically rejects and strongly condemns” the “unfounded and malicious speculations” that Tehran was involved.

No charges, and doubts over a terror motive

Some security analysts said the decision to bail the men, rather than charge them or bring them before a court, pointed away from a live terrorist plot. Chris Phillips, a former head of Britain’s National Counter Terrorism Security Office, told Al Jazeera that “if there had been any explosives, big or small, in those vans, then there is no way on Earth that the police would have released these individuals on bail.”

A resident of Kempsford, a village near the base, told AFP the incident had proved “a lot of noise for nothing,” while another local, who declined to give her name, said, “If they were true terrorists they wouldn’t have released them.”

Aerial footage showed the vans contained barrels, with at least one marked with a false logo and phone number designed to look like it belonged to a legitimate diesel delivery firm. That company, Fuel2U, said in a statement that “neither the vehicle nor any of the individuals involved has any connection whatsoever” with it, and that it was cooperating with police.

A base at the centre of the Iran war

RAF Fairford, roughly 160 kilometres west of London, is the only airfield on Britain’s mainland used by U.S. heavy bombers for operations against Iran. Then-Prime Minister Keir Starmer agreed in March to let Washington use British bases for what he called “defensive” operations against Iranian missiles and launch sites, after Iranian attacks on British interests and allies in the Gulf. American B-1 and B-52 bombers have flown from Fairford since the Iran war began on Feb. 28.

Iran has said it considers any base used to strike its territory a legitimate target. British security officials have said proxies linked to Tehran have carried out a string of arsons and other attacks in the U.K. and elsewhere in Europe, many targeting the Jewish community, and have warned Iran could widen its targets if the conflict continues.

British Foreign Secretary Ed Miliband said the incident was “a reminder of the threats our country faces” and thanked security services, the armed forces and police for their work. Security at Fairford had reportedly been increased in the weeks before the arrests.

Counter Terrorism Policing said the roughly 85 households evacuated near the scene were being allowed to return, with the police cordon expected to close entirely by Wednesday. Taylor said the five men remain under investigation as detectives pursue “multiple lines of enquiry.”