President Donald Trump has said a suspected terror plot against an airbase used by the U.S. Air Force in the U.K. was “looking to do big damage”.

Five men were arrested on Sunday on suspicion of explosive and terrorism offences, after armed police stopped three vans in the early hours of the morning near RAF Fairford in Gloucestershire.

Trump hailed the U.K. police response, while suggesting authorities already had the suspects under investigation.

Speaking to reporters in the U.S., Trump said: “It was fantastic. Working with Britain, it was an amazing job.”

“We had them under investigation,” the president continued. “They, they were looking to do big damage to our fort, and working with the British worked out great.

“It may have been Scotland Yard, I don’t know. We worked with a lot of them. We had them under view for a long time, and we got them. Thank you.”

Three “suspicious vehicles” travelling towards Fairford were reported to police at approximately 12.45a.m. on Sunday. The men were subsequently arrested by armed officers under suspicion of committing offences under the Explosives Act.

They were then further arrested under suspicion of preparation of a terrorist act.

Fairford has been raised to the highest threat level, Delta. Another base, RAF Lakenheath some 150 miles away, is on Charlie alert, the second-highest threat level that can be issued by the Department of Defense.

A spokesperson for the USAF said its personnel based at Fairford “remain vigilant”, but “for operational security reasons we will not discuss specific force protection measures”.

They said its 501st Combat Support Wing and U.K. counterparts “will take appropriate actions to ensure the safety and security of our U.S. 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.

A local resident told the Press Association she called police after she saw three vans “blocking the road”, before spotting a “large group of hooded and ‘balaclava’d’ men”.

The woman, who asked not to be identified, said she was driving home in the early hours of Sunday morning when she saw 10 people in the road.

The resident said the RAF Fairford base was known by residents to be on lockdown due to a “terror threat”.

She called the emergency services after discovering the men and said police arrived on the scene around 15 minutes later.

About 85 households were evacuated from their homes near RAF Fairford after a major incident was declared by police. A large emergency services presence was seen in the area, including a Hazardous Area Response Team within a 400-metre cordon.

Aerial footage showed a bomb disposal robot inspecting three white vans, two of which had open rear doors. Large black barrels appeared to be scattered around.

Counter Terrorism Policing senior national co-ordinator, Deputy Assistant Commissioner Vicki Evans, said: “We are in the early stages of an investigation and five men remain in custody.

“As you would expect, we are working closely with our partners to understand the circumstances of the incident.

“We continue to work with Gloucestershire Police to keep the community safe as we work to gather evidence from the scene.”

Suspects arrested under the Terrorism Act 2000 in the U.K. can be detained for up to 14 days without charge, compared to the 24-hour limit for standard arrests without an extension application.

The arrests come amid heightened concerns around the threat from hostile states to the country, with Russia and Iran among those that have used proxies to commit crimes on their behalf on British soil.