Counter terrorism police are continuing to examine a scene near an RAF base after releasing five suspects on bail over a suspected terror plot.
Five British nationals were arrested on suspicion of explosives offences and terrorism on Sunday after three vans were found near RAF Fairford in Gloucestershire, which is used by the US.
Residents evacuated from about 85 households were able to return home on Monday, but a cordon remains in place around the vehicles.
It is understood that the material found was not viable explosives, but police have said the investigation is continuing.
The head of UK Counter Terrorism Policing, Assistant Commissioner Laurence Taylor, acknowledged speculation over whether Iran was behind the plot and said the force was considering “every possible angle”.
In a statement outside New Scotland Yard on Monday, Mr Taylor said he knew there was “significant interest in whether the current geopolitical context has a bearing on this incident”.
He said: “At Counter Terrorism Policing, we are working within this context every single day and, at this stage, we are keeping an open mind in this investigation.
“We have, as you would expect, ensured that our protective security advice and posture remain commensurate to the threat.
“But be assured, we are considering this from every possible angle, including that this may be actively committed by proxies or individuals either knowingly or unknowingly working on behalf of a foreign state.”
Iran, and other hostile states including Russia, has been known to use criminal proxies to carry out crimes on its behalf on UK soil.
Its London embassy earlier said it “categorically rejects and strongly condemns the recent unfounded and malicious speculations” of Iranian involvement.
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.
Mr Taylor said the men were released on bail under “stringent conditions” on Monday but “absolutely remain under investigation as we explore multiple lines of inquiry”.
Prime Minister Andy Burnham chaired a Cobra meeting on Monday.
After the meeting, Defence Secretary Wes Streeting said the decisions around bail were operational matters for the police and prosecutors, and the investigation remained ongoing.
US President Donald Trump said he was “surprised” the men were released on bail.
Speaking in the Oval Office on Monday, Mr Trump said: “I will tell you, I would have said ‘Don’t let them have bail’. I’m surprised that they released them. I wouldn’t have done that.”
Gloucestershire Constabulary was initially alerted to three suspicious vans travelling towards RAF Fairford at 12.45am on Sunday, and it is unclear what action was taken in the hour that followed.
