Police discovered a quantity of petrol but no explosive devices in the vans that sparked a major security alert near an RAF base used by the US, police have said.

A terror probe was launched after police were alerted to a suspected “bomb plot” at RAF Fairford on Sunday. A 400-metre cordon was thrown around the scene as bomb disposal experts examined three vans at the centre of the counter-terrorism police investigation.

Five men who were arrested on suspicion of explosives offences and later terrorism were all released on bail on Monday, which head of UK counter-terrorism Policing Assistant Commissioner Laurence Taylor said was an investigative decision.

Speaking outside New Scotland Yard, Assistant Commissioner Laurence Taylor confirmed on Tuesday that no improvised explosive devices had been found within the vans.

“A police response was activated in the early hours of Sunday morning due to a range of information that we received,” he said. “This included a call from the member of public about several vehicles acting suspiciously close to RAF Fairford.

“Now, those vehicles, as I explained yesterday, have been subject to extensive examination, with the support of military assets and forensic experts, and we can confirm that no improvised explosive devices have been found.

“We have found a quantity of an accelerant, petrol, and we have recovered that as part of our investigation.”

He added the five suspects released on bail on Monday “remain under investigation” and “must adhere to strict conditions” as the probe continues. “The safety of the public is at the forefront of our investigative decision making, and this was certainly the case yesterday,” he said.

“I understand why this development will have come as a surprise to many, but I’d like to reiterate that it was investigative decision, and one which is grounded in experience and careful consideration of policing powers.”

The five suspects are a 24-year-old from Camden, north London; three men aged 23, 24 and 25 from Westminster, central London, and one aged 24 from Willesden, north-west London.

Villagers living near RAF Fairford in Gloucestershire, dozens of whom were dramatically evacuated from their homes on Sunday morning, were slowly returning to normal on Tuesday. Police cordons are expected to be fully lifted on Wednesday.

Questions over Iran’s potential involvement in the attack have continued as US secretary of state Marco Rubio claimed in a television interview that the suspected plot “clearly involves the hand of a foreign actor”.

Speaking to Fox News, Mr Rubio said: “It’s one that clearly involves the hand of a foreign actor. I won’t get into great detail about that yet.

“I do want to thank the authorities in the United Kingdom who have been very co-operative and on top of it.”

He added: “I know a lot of people are disturbed by the news that some of these people have been released on bail, as some of the people are being investigated, and obviously we’re engaging with them on all of that.

“Beyond that, I would argue that they’ve been very co-operative and very helpful on something that’s a very serious incident.”

It comes after US president Donald Trump said he was “surprised” the suspects had been released on bail.

Iran’s embassy in London has strongly denied any involvement in the alleged plot.