British counterterrorism police have arrested a 27-year-old dual UK-Iranian national in connection with the investigation into a suspected plot near RAF Fairford, the Gloucestershire airbase used by US forces in the war on Iran.

The man was arrested on Thursday afternoon in the London borough of Westminster on suspicion of preparing terrorist acts under Section 5 of the Terrorism Act 2006.

Police said a second man, a 26-year-old UK national, was also questioned under caution, and officers searched two properties in the same area. Five men in their 20s who were arrested on Sunday remain on police bail. Vicki Evans, the Senior National Coordinator for Counter Terrorism Policing, described the investigation as “hugely complex”, with specialist teams working on multiple lines of enquiry, including possible foreign state involvement.

The case began in the early hours of Sunday, when residents in the village of Whelford alerted emergency services to three suspicious vehicles heading towards the base.

Armed officers arrested five men, all British nationals aged between 23 and 25 and living in London, on suspicion of Explosives Act offences and the preparation of a terrorist act. A 400-metre cordon was set up, about 85 households were advised to evacuate, and bomb disposal teams were deployed.

Counter Terrorism Policing later took over the inquiry. RAF Fairford hosts the US Air Force’s 501st Combat Support Wing and was used to deploy B-1 and B-2 bombers in strikes on Iran.

Politicians have been more forthcoming than the police about who may be behind the plot. US Secretary of State Marco Rubio said the incident “clearly involves the hands of a foreign actor”, while President Donald Trump said the suspects were looking to do “big damage” and had been under watch for some time.

Prime Minister Andy Burnham said there are strong indications that Iran played a part. Defence Secretary Wes Streeting earlier said it would be irresponsible to speculate.

Tehran has denied any link. Iran’s embassy in London called the claims unfounded and malicious. On Thursday, Iran’s foreign ministry summoned British Ambassador Hugo Shorter to protest what it called baseless accusations of involvement in a sabotage plot.

Deputy Foreign Minister, Majid Takht-Ravanchi also accused Britain of complicity in US and Israeli military action by allowing its bases to be used.

No one has been charged so far, and police have not said what the vehicles were carrying. Questions also remain over how the group got so close to the base before a member of the public raised the alarm.