The five men, all U.K. nationals in their 20s from London, were arrested Sunday on suspicion of terrorism and explosives offenses after three vehicles were seen moving toward RAF Fairford, a Royal Air Force station about 90 minutes west of London.

Counterterrorism police took over the case after a major incident was declared, disrupting life in the normally quiet rural community of the Cotswolds.

On Monday, British counterterrorism officials running the operation said the men have been released on bail after “extensive interviews and inquiries.” The investigation was not over, however, the head of Counter Terrorism Policing, Laurence Taylor, said, and “multiple lines of inquiry” were still being probed.

No charges have been filed against the men.

Taylor said the three vans that the suspects were travelling in before the arrests have been “subject to extensive examination with the support of military assets and forensic experts.”

He did not say if any explosives have been found during the searches, as the question remains about what was inside the vans. According to several British reports, no explosives were found.

NBC News reached out to the counterterrorism police for clarification on Tuesday but did not receive a response.

On Sunday, counterterrorism police said the men had been arrested on suspicion of committing offences under the Explosives Act, U.K. legislation dating back to 1875 that regulates manufacturing, keeping, selling, carrying and importing of gunpowder and explosive substances. The men were further arrested on suspicion of preparation of a terrorist act under Terrorism Act, which deals with terrorism-related offenses.

The incident prompted has questions about whether a foreign actor could be involved — in particular Iran, after the country’s Islamic Revolutionary Guard Corps warned Britain this summer that RAF Fairford could be a “legitimate target.” It came months after then-Prime Minister Keir Starmer gave permission for the U.S. to use its bases for “specific and limited defensive” operations against Iran.

Iran’s embassy in the U.K. has vehemently denied any involvement on Monday.

During an appearance on Fox News later on Monday, Rubio said that “what happened, what almost happened, what could have happened in the U.K. over the weekend, is a very serious situation.”

“There’s a lot more news that’ll come out on this in the days to come,” he added.

Taylor, with U.K.’s counterterrorism police, noted there had been “significant interest in whether the current geopolitical context has a bearing on this incident.”

The investigators are keeping an open mind, he said.

“Be assured we are considering this from every possible angle, including that this may be actively committed by proxies or individuals either knowingly or unknowingly working on behalf of a foreign state,” he added.