BBCFive men have been arrested on suspicion of explosive and terrorism offences after a major incident was declared at an airbase in the west of England.
Bomb disposal teams have been examining a number of vehicles near RAF Fairford in Gloucestershire and dozens of homes in the surrounding area have been evacuated.
Here is a summary of what we know so far.
Where is RAF Fairford?
The base shares a long history with the American military, and has recently been used by US warplanes as a staging post for defensive strikes on Iran.
What happened?
Local police said they received a call about “three suspicious vehicles” travelling towards RAF Fairford at 00:45 BST on Sunday.
Five men were later arrested by armed officers near Whelford, a Cotswold village around two miles (3.2km) south-east of the base.
A bomb disposal team was also deployed to the village to examine a number of vehicles, around which a 400m (1,312ft) cordon has been placed.
Aerial pictures showed a remote-controlled robot approaching two stationary white vans in a wooded area, though its use has not been confirmed by police.
There has been large emergency service presence in the area since the early hours of Sunday, which Gloucestershire Police Assistant Chief Constable Richard Ocone said would remain “for some time”.
Ocone told a news conference on Sunday afternoon that “work to establish the full nature of the incident is ongoing”.
Counter Terrorism Policing are now leading the investigation. Deputy Assistant Commissioner Vicki Evans said officers were “working closely” with partners to “understand the circumstances of the incident”.
Who are the suspects?
The five men were initially detained on suspicion of committing offences under the Explosives Act.
They were then further arrested on suspicion of preparation of a terrorist act.
The men, who remain in custody, have not been named.
US President Donald Trump said those arrested were “looking to do big damage”, adding: “We had them under view for a long time, and we got them.”
BBC Your Voice has since received an image purporting to show the moment five men were arrested.
It showed detained men in Whelford, about 350m (1148ft) from runway access at Crash Gate 6A at RAF Fairford.
The BBC has been unable to verify the original source but BBC Verify has geolocated the image and confirmed it appeared to be authentic.
How many homes have been evacuated?
Eighty-five households have been evacuated and residents have been urged to shelter at a nearby leisure centre.
Several people have told the BBC they were woken by police in the middle of the night and ordered to leave their homes.
“We just had to throw our clothes on and come. We didn’t know anything,” one resident, who named herself only as Jo, said.
Another resident, Jason, said: “It all happened very quickly. We just packed our stuff up and came here.”
Several people in the leisure centre are still wearing their pyjamas, according to a BBC reporter there.
Gloucestershire County Councillor Paul Hodgkinson has told PA news agency there is a chance that the evacuated residents may have to stay overnight while the area around their homes is made safe.
What have eyewitnesses said?
One Whelford resident has described the moment he said he saw police apprehend the suspects.
Jeff told the BBC he heard “shouting outside his front door”, although he could not be sure where it came from.
He said he then looked out of his window and saw “people being held by police outside”.
Another man, Julian Tudsbery, said: “There were these three vans that arrived in the middle of the village and a resident reported them to the police straight away.”
