One of five men arrested near RAF Fairford in the early hours of Sunday morning had dialled 999 himself, the BBC understands.

The suspect called the police at 00:45 BST, almost an hour before a female farmer also dialled 999 after seeing men in balaclavas.

The five were arrested on suspicion of explosives offences and later terrorism offences after three vans were spotted, sparking a major security incident around the Gloucestershire base which the US military operates.

They were released on bail on Monday in what police described as an “investigative decision” “grounded in experience and strategy”.

Police say a quantity of petrol but no explosive devices were found during searches of the vans.

Police were alerted to “three suspicious vehicles” in Whelford, a village which lies directly east of the base, by the farmer.

She told the BBC it was “pure luck” she had spotted them, having arrived home to find the road blocked and a “large group of hooded and masked men”.

“I believe that my 999 call foiled whatever plans they had,” she said.

A bomb disposal team arrived on scene shortly afterwards and a 400m (1,312ft) cordon was put in place.

But Assistant Commissioner Laurence Taylor, head of Counter Terrorism Policing, told reporters on Tuesday that the police response that morning had been due to “a range of information”.

Police cordons near the base are expected to be completely lifted on Wednesday, while residents from the 85 homes evacuated on Sunday have been allowed to return to their properties.

The five men are all UK nationals aged between 23 and 25. Four of them live in Westminster and one in Camden, and searches of their homes are now complete.

Taylor previously said their bail was “subject to stringent conditions on their movement and contact with others” as they remained under investigation.

“The safety of the public is at the forefront of our investigative decision-making, and this was certainly the case yesterday,” he said on Tuesday.

“I understand why this development was surprising to some, but I would like to reiterate that this is an investigative decision and one which is grounded in experience and strategy.”

The RAF base has been used as a forward operating location by the US Air Force 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.

US President Donald Trump on Tuesday told journalists he was “disappointed” the five men had been released on bail.

Taylor has said officers are exploring “multiple lines of inquiry”, including whether proxies or individuals knowingly or unknowingly working on behalf of a foreign state were involved.

In an update on Tuesday, he added that the incident had been worrying for many people, with this “heightened by the international commentary that we are seeing” in an apparent reference to comments from the US administration.

Prime Minister Andy Burnham chaired a Cobra meeting on Monday afternoon and urged people “not to speculate while the authorities establish the full facts”.

In July, Iran said “any base used for aggression against Iranian territory constitutes a legitimate target for our forces”.

However, the Iranian embassy in London has said it “categorically rejects and strongly condemns” “unfounded and malicious speculations” linking Iran to the latest incident.