Five men who were arrested on suspicion of terrorism offences near RAF Fairford are being released on police bail, counter-terror police have said.
A major incident was declared and dozens of homes were evacuated after police were alerted to three "suspicious vehicles" near the airbase in Gloucestershire in the early hours of Sunday morning.
Counter Terrorism Policing (CTP) said officers were exploring "multiple lines of inquiry", including whether proxies or individuals working on behalf of a foreign state were involved.
The five men are all UK nationals in their 20s from London.
They were initially detained on suspicion of committing offences under the Explosives Act and then further arrested on suspicion of preparation of a terrorist act.
Counter-terror police said they remained under investigation and their release was "subject to stringent conditions on their movement and contact with others".
Three of the men live in Westminster, one in Camden and another in Willesden, police said, and they are aged between 23 and 25.
A farmer who called 999 to report the three white vans in the village of Whelford, which lies directly east of the base, told the BBC it was "pure luck" that she spotted them.
The woman, who wished to remain anonymous, said she arrived home to find the driveway and road blocked and saw a "large group of hooded and masked men".
"When they saw me they ran ahead of me - some into the field and some towards the neighbouring village of Kempsford. I then realised that something highly unusual was taking place," she told BBC Radio 4's Today programme.
She said that she called the Ministry of Defence police at RAF Fairford but received no answer. She then rang 999 at 01:36 BST and said "within 10 minutes the village was full of armed police".
"I believe that my 999 call foiled whatever plans they had," she added.
Assistant Commissioner Laurence Taylor, head of Counter Terrorism Policing, said on Monday afternoon that the three "suspicious vehicles" had been "subject to extensive examination, with the support of military assets and forensic experts".
Officers were considering the incident "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", he added.
Prime Minister Andy Burnham chaired a Cobra meeting on Monday afternoon and urged people "not to speculate while the authorities establish the full facts".
The Iranian embassy in London said it "categorically rejects and strongly condemns" what it called "unfounded and malicious speculations" linking Iran to the incident, adding that these would "serve no purpose other than to fuel Iranophobia propaganda in the UK".
The embassy said Iran "reserves the right to take appropriate legal measures, as necessary, to safeguard the rights and interests of the country and its people in response to misleading and damaging media narratives".
The RAF base has been used as a forward operating location by the US Air Force (USAF) for decades.
In recent months, US bombers have taken off from the base for the Middle East, after the UK approved a US request to use it for defensive strikes on Iran.
In July, Iran said "any base used for aggression against Iranian territory constitutes a legitimate target for our forces".
Late on Monday, US Secretary of State Marco Rubio told a TV show that it was clear a "foreign actor" had been involved in events over the weekend.
Without providing evidence or giving further details, he told Fox News network's "Hannity" programme: "What happened, what almost happened, what could have happened in the UK over the weekend is a very serious situation. It's one that clearly involves the hands of a foreign actor."
Speaking earlier at the Oval Office, US President Donald Trump said he would not have let the five men arrested out on bail, and that he was "surprised" they had been released.
"We know everything about them, we would not have released them," he said, adding that the UK had worked with the US "very closely" on this.
A large cordon remains in place near the base, while 85 households who were evacuated on Sunday have now been told by Gloucestershire Police that they can return home.
But police have said a cordon will remain around the three vehicles.
Gloucestershire Constabulary's Temporary Chief Constable Maggie Blyth said the last 48 hours had thrown "our usually quiet and peaceful communities under the spotlight of the international media".
Airspace restrictions have also been put in place above the base - nobody can fly within a five mile (8km) zone encircling it without permission. Banned items include balloons, kites, parachutes and paragliders.
