The US government has described the security alert over the weekend at the RAF Fairford base in southern England as a "very serious situation," thanking UK authorities for their cooperation.
But President Donald Trump and Secretary of State Marco Rubio also both questioned the move to grant the five suspects bail under strict conditions as investigations against them continue.
RAF Fairford is a major military air field fairly close to the south coast of England. It's a rare European base from which the US operates heavy bombers, with the range to reach Iran amid the current conflict in the Gulf.
What did Rubio and Trump say about the case?
Marco Rubio said in an interview with Fox News that while there was much he could not say on air for now, the security scare during which five young men were detained constituted a very serious situation.
"It's one that clearly involves the hand of a foreign actor. I won't get into great detail about that yet," he said, without mentioning Iran until a later point in the interview in a more hypothetical context.
"I think it ties us to the reality here, and that is we are dealing with elements in the world, and in the case of Iran, for example, who have openly threatened to attack American interests globally," Rubio said.
The Iranian Embassy in London, meanwhile, said that it "categorically rejects and strongly condemns the recent unfounded and malicious speculations" of Iranian involvement.
Rubio also echoed President Donald Trump in saying that "a lot of people are disturbed by the news that some of these people have released on bail."
Trump had mentioned the issue in comments to reporters in the White House earlier on Monday.
"I will tell you, I would have said, 'Don't let them have bail.' I'm surprised that they released them. I wouldn't have done that," he said.
What have British investigators said about the case?
Five men in their 20s, all from the London area and with British citizenship, were arrested on Sunday in the vicinity of the air base, which is in a rural area roughly two hours' drive west of the capital. They were wearing face coverings.
Police seized three "suspicious" white vans and evacuated around 85 households in the vicinity, bringing in bomb disposal teams.
Locals were able to return home on Monday, and preliminary reports from the British Press Association and others suggestd the materials in the vehicles were not viable explosives, but counterterrorism and explosives experts continued to examine the scene.
The head of UK Counter Terrorism Policing, Assistant Commissioner Laurence Taylor, acknowledged the "significant interest in whether the current geopolitical context has a bearing on this incident," and said investigators were considering "every possible angle."
One of these angles, he said, was the possibility "that this may be actively committed by proxies or individuals either knowingly or unknowingly working on behalf of a foreign state."
Taylor said that the men had been released under stringent conditions, and that granting them bail did not in any signify an end to investigations against them.
Defense Secretary Wes Streeting said that decisions like granting bail were operational matters for police and prosecutors, not politicians.
Prime Minister Andy Burnham chaired a security meeting known as Cobra, after the secure Cabinet Office Briefing Room A that hosts such sessions, on Monday evening.
What is RAF Fairford?
RAF Fairford is a large military airbase not far north of Swindon in Gloucestershire, west of London.
It's a base for US heavy bombers in Europe and has been used as a launch point for operations involving long-range B-52s in a series of conflicts in recent decades including both Iraq wars, the NATO bombing of the former Yugoslavia, and most recently in a limited capacity in Iran this year.
Residents in the Cotswolds region had noted an increased security presence and activity at the base amid the conflict, as is often observable around such bases during such periods.
Because of the longstanding close military ties — and very often infrastructure set up during World War II when Nazi Germany occupied much of Europe — southern England is home to a string of airbases also used by the US military.
