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”.
Speaking at the Oval Office on Monday, US President Donald Trump said he wouldn’t 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.