British counter-terrorism police investigating a suspected plot involving an air base used by the US said on Saturday they had released on bail a dual UK-Iranian national who had been arrested on suspicion of preparing terrorist acts.

The 25-year-old man was detained on Thursday over the incident last weekend involving the RAF Fairford base in western England from where US bombers have taken part in raids against targets in Iran.

Police confirmed he had now been released, and searches at two properties in London had concluded.

“The investigation into the circumstances surrounding events in Gloucestershire is continuing,” police said in a statement.

The incident near Fairford began in the early hours of last Sunday when five British men in their 20s were arrested near three vans on suspected terrorism and explosive offences.

The men were later released without charge on police bail “under stringent conditions”, and although detectives said petrol was found in the vans, no improvised ​explosive device was present.

Counter-terrorism police, who have described the investigation as complex, said they were considering whether a foreign state had been involved and Prime Minister Andy Burnham said on Wednesday it might have had an Iranian link.

US President Donald Trump has said there was a plan to cause “big damage” at the air base, while US Secretary of State Marco Rubio also pointed to the “hands of a foreign actor” in the incident.

Iran has said accusations it was involved were baseless and on Thursday summoned the British ambassador in Tehran over the issue.