US President Donald Trump said he was "surprised" after British police announced they were releasing five men arrested on suspicion of terror offences on bail.
"I'm surprised that they released them, I wouldn't have done that, " he told reporters on Monday, adding that the case "might be" linked to Iran.
"We know everything about them. We would not have released them," he went on.
Laurence Taylor, the head of UK counter-terrorism policing, said earlier that the men were to be released on police bail following "extensive interviews and inquiries."
"This does not mean that the investigation is over," he said, adding that they would be "subject to stringent conditions on their movement and contact with others."
"They absolutely remain under investigation as we explore multiple lines of inquiry," Taylor continued.
British counter-terrorism police are investigating whether there is a link to Iran after police found explosives on three vans near an RAF base in Gloucestershire on Sunday.
Speaking to the BBC on Monday morning, Defence Secretary Wes Streeting cautioned that it would be "irresponsible to speculate."
"As soon as we can keep the public informed, we will. I would just ask people for a bit of forbearance to give the police and security services the time and space to do their job," he said.
Since March, RAF Fairford has been used by the US military to launch air strikes on Iran as the war in the Middle East rumbles on.
In July, the Islamic Revolutionary Guard Corps (IRGC) said it was aware that the base was being used by US bombers and warned the UK that “any base used for aggression against Iranian territory constitutes a legitimate target for our forces.”
Armed officers detained the group of five men on Sunday after a call to emergency services about three "suspicious vehicles which appeared to be travelling towards the air base," London's Metropolitan Police said in a statement.
Counter-terror police said on Monday that all the men are British nationals in their mid-20s.
US President Donald Trump told reporters they had been "looking to do big damage" at the base in southwest England.
"We had them under view for a long time, and we got them," he said, adding the US and UK had been working together on the incident.
A local resident who said it was known locally that the base was on lockdown due to a "terror threat" alerted emergency services after seeing three vans blocking the road and a large group of men wearing hoods and balaclavas.
She told the PA news agency she found it "astonishing" it had been effectively down to a "plucky amateur" spotting unusual activity to raise the alarm.
Some 85 households in the nearby village of Whelford were advised to evacuate and army bomb disposal experts called in to examine three large white vans stopped on a rural road.
RelatedIncident 'contained'
"Counter Terrorism Policing can now confirm they are leading an investigation into an incident which took place overnight near RAF Fairford in Gloucestershire," the Met statement added.
Broadcast aerial footage showed a bomb disposal robot moving around the vans, two of which had their rear doors open.
Police believed the incident had been "contained" and a 400-metre cordon had been put in place.
Armed officers could be seen guarding the entrance to the base.
In March, then prime minister Keir Starmer granted permission for "defensive" US action against Iranian missile sites from two UK bases including Fairford.
New Prime Minister Andy Burnham was being kept updated on the incident, a government spokesperson said.
Burnham, who was in the northwestern city of Liverpool along with other senior ministers for his Labour Party's annual conference, thanked the police, armed forces and residents who had been asked to leave their homes "while precautionary checks are carried out."
Attacks by proxies
Fairford is the UK base of the 501st Combat Support Wing, a US air force administrative support wing, which said in a statement that no US air force personnel had been involved in the incident.
"The 501st Combat Support Wing appreciates the swift response and continued support of our local law enforcement partners," it said.
"We remain in close coordination with our UK partners to help ensure the safety and security of both our military and local communities," it added.
The incident comes amid concerns about the actions of hostile foreign states.
Interior Minister Shabana Mahmood has accused Iran and Russia of using proxies to carry out criminal activities on their behalf on UK territory.
Earlier this month, reports said British military sites had been targeted by drones more than 300 times over the past year.
