Counter Terror Police are understood to be investigating whether Iran is linked to a suspected terror plot at a military airbase used by US bombers.
Five men were arrested on suspicion of explosive and terrorism offences after armed police stopped three vans travelling towards RAF Fairford.
The group, detained in the nearby village of Whelford, Gloucestershire, in the early hours of Sunday, were “looking to do big damage”, US President Trump later said.
The police operation was not intelligence-led, meaning investigators had not known about any potential plot in advance, the Press Association understands.
A resident dialled 999 after spotting the vehicles “blocking the road” before seeing a “large group of hooded and ‘balaclava’d’ men”.
The Whelford resident told the Press Association “they would have seen the bright headlights” and “they probably thought we were the police (and) panicked”.
She added: “It would appear that by doing so I just derailed whatever they were planning.”
Officers arrived on the scene about 15 minutes after she raised the alarm, she said.
Residents had at the time understood the base was on lockdown because of a “terror threat”, the woman added.
“A plucky amateur shouldn’t be spotting an unusual bit of activity at 1am when the air base is on high alert,” she went on. “I think it’s absolutely astonishing.”
A pensioner living nearby similarly described an apparent increase in Fairford’s security in recent weeks, telling PA that heavy tractor-like vehicles had been parked and concrete walls laid behind the gates.
Gloucestershire Constabulary said it received reports of the suspicious vehicles travelling to the base at about 12.45am on Sunday.
Officers arrested the men under the Explosives Act, and they were later further arrested under suspicion of preparation of a terrorist act.
Suspects detained under the Terrorism Act 2000 can be held for up to 14 days without charge, compared to the 24-hour limit for standard arrests without an extension application.
Aerial footage showed a bomb disposal robot spending hours inspecting three white vans, two of which had open rear doors. Large black barrels appeared to be scattered around.
A 400-metre cordon was erected, and 85 evacuated households were unable to return home overnight. Cotswold District Council was seeking accommodation for some.
The army’s ordnance disposal unit was still examining the vehicles in the cordon by the evening, Temporary Chief Constable Maggie Blyth, from Bedfordshire Constabulary, told a press conference.
Agencies “believe the matter to be contained”, she later added.
Prime Minister Andy Burnham said the incident was “deeply concerning” and the government would “always take every step necessary” to protect people.
Investigators will now be working to analyse any devices seized from those arrested to establish details of any plot and any potential motivation.
The arrests come amid heightened concerns around the threat from hostile states to the UK, with Russia and Iran among those that have used proxies to commit crimes on their behalf on British soil.
In March, officials said the number of national security cases involving activity by hostile states, including Iran, increased by 50 per cent in the six months to December 2025.
That same month, after then-Prime Minister Sir Keir Starmer granted permission for “defensive” US action against Iranian missile sites from UK bases, four American bombers landed at RAF Fairford and the US started using British bases for “specific defensive operations”.
Separately, a spate of antisemitic attacks on Jewish sites in London this year were claimed by a group called Harakat Ashab al-Yamin al-Islamia (Hayi), which is thought to be backed by Iran.
Support for Iran’s Islamic Revolutionary Guard Corps (IRGC) and Hayi was banned in July by the Home Office.
Under new state threat powers, anyone supporting or assisting the groups will face a maximum punishment of 14 years in prison, but ministers will need parliamentary approval first.
The Fairford incident also took place against the backdrop of ongoing war between the US and Iran.
RAF Lakenheath in Suffolk confirmed it was on Charlie alert, which is the second-highest threat level that can be issued by the US Department of Defence.
The confirmation to PA came amid reports that RAF Fairford has been raised to the highest threat level, Delta.
At a fringe event at the Labour Party conference, Defence Secretary Wes Streeting was asked if there was any Iranian involvement in the suspected plot.
He said: “I’m not going to comment on the underlying motivations of those who’ve been arrested today, or indeed the, sort of, the genesis of their activity. I think it’s far too early to comment or speculate, and it would be irresponsible of me to do so.”
A spokesperson for the US Air Force said its personnel based at RAF Fairford “remain vigilant”, but “for operational security reasons we will not discuss specific force protection measures”.
They said its 501st Combat Support Wing and UK counterparts “will take appropriate actions to ensure the safety and security of our US service members, civilians, contractors, and their families”.
The 501st is “the only combat support wing in the USAF” and operates from 10 bases, including Fairford, according to its website.
Trump, meanwhile, praised the UK response and suggested authorities already had the suspects under investigation, despite others’ claims to the contrary.
Using the American term for military sites – fort – to refer to RAF Fairford, which hosts USAF heavy bombers, he told reporters in the US: “They were looking to do big damage to our fort, and working with the British worked out great.
“We had them under view for a long time, and we got them. Thank you.”
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