British police said Monday five men arrested near an airbase used by the United States were released on bail, as Washington asserted a "foreign actor" was involved in the suspected bomb plot.
US President Donald Trump, speaking in Washington, said he was "surprised that they released them", adding that "I wouldn't have done that".
The release of the five -- all London-based Britons in their mid-20s -- came as British Prime Minister Andy Burnham announced an emergency meeting of top officials to ensure they have "the full picture".
Earlier, Iran had denied involvement in any plot to target the Royal Air Force (RAF) Fairford base in Gloucestershire, southwest England, following the security scare there early Sunday.
Here's what we know so far:
UK counter-terror police are leading the investigation after authorities received reports Sunday of three "suspicious" white vans near the airbase, which has been used by the United States to bomb Iran.
Police said in their latest update Monday that the five men held, who all live in inner London boroughs, were being released on bail.
"This does not mean the investigation is over," Assistant Commissioner Laurence Taylor, the head of Counter Terrorism Policing, said, adding the five would be subject to "stringent" bail conditions.
He said officers were "considering this from every possible angle, including that this may be activity committed by proxies, or individuals either knowingly or unknowingly, working on behalf of a foreign state".
The men were arrested on suspicion of explosives offences and "preparation of a terrorist act" under UK anti-terrorism laws, according to police.
They had been travelling in the three suspect vehicles, which have since been subject to "extensive examination, with the support of military assets and forensic experts".
According to AFP, authorities have not confirmed details of the threat but bomb-disposal experts were seen working at the scene.
The Daily Mail and Press Association both reported late Monday they understood from sources that tests on materials in the back of the vans established they were "not viable explosives".
A farmer living in the village of Whelford near the airbase said she rang the police after seeing the vans blocking a road and a group of masked men.
The anonymous woman told media on Monday that she believed the police had "dropped the ball" on the threat.
But UK Defense Minister Wes Streeting said the farmer only had a "partial picture".
Trump said on Sunday that Britain and United States had had the suspects "under investigation" for a while and alleged "they were looking to do big damage to our fort".
After hearing news of their bail, Trump said Monday the case "might be" linked to Iran but declined to elaborate.
US Secretary of State Marco Rubio said the incident "clearly involves the hands of a foreign actor", without giving further details.
Iran's embassy in London said on X that it "categorically rejects and strongly condemns" the "unfounded and malicious speculations" that Tehran was involved.
Britain gave the United States permission in March to use Fairford for "specific defensive operations into Iran" to destroy Iranian missiles at source.
US bombers and other planes have since been seen taking off and landing from the base -- the only one on Britain's mainland used for this purpose.
Residents in the rural Cotswolds area surrounding Fairford said the site, which hosts a range of US military equipment and personnel, had seen increased activity and security in recent months.
Authorities evacuated around 85 households from the village early on Sunday, and they were allowed to return home the next day.
Tehran has previously warned London against allowing its bases to be used by the United States in operations against Iran.
Iranian Foreign Minister Abbas Araghchi has accused London of "putting British lives in danger" by allowing the bases to be used "for aggression against Iran".
