A UK-Iranian dual national has been arrested in Westminster, London, on suspicion of preparing for terrorist acts in connection with an incident at RAF Fairford, counter-terror police announced. The 25-year-old man's arrest follows an investigation into events at the Gloucestershire airbase on Sunday, which led to a major security alert.

Two properties in the Westminster area have been searched as part of the ongoing investigation. This development comes after five UK nationals, all in their twenties and from London, were initially arrested near the airbase on Sunday. They have since been released on police bail.

Counter-terror police are examining all potential motives, including the possibility of foreign state involvement, according to Vicki Evans, the force's senior national coordinator. She described the investigation as "hugely complex" with "multiple lines of enquiry" and thanked the public for their support.

Prime Minister Andy Burnham stated on Wednesday that there were "strong indications" of Iran's involvement in the incident, though he provided no further details. Tehran has issued a strong denial of any participation.

The initial arrests occurred early Sunday morning after three suspicious vehicles were reported heading towards the US Air Force-operated base. The group, aged between 23 and 25, were first detained on suspicion of explosives offences before the suspicion shifted to preparing a terrorist act. Police stated their release on bail was an "investigative decision grounded in experience and strategy."

Searches of the vans near the RAF Fairford site revealed a quantity of petrol but no explosive devices. Four of the five UK nationals reside in Westminster, with one living in Camden. Their homes were also searched.

US Secretary of State Marco Rubio commented on Monday that the involvement of a "foreign actor" was "clearly" evident in the incident at the base, which serves as a forward operating location for the USAF. Recent months have seen US bombers departing from the base for missions in the Middle East, following UK approval for defensive strikes against Iran.

On Thursday, the British ambassador to Iran met with representatives from Iran's foreign ministry at the request of the Iranian government, signalling diplomatic engagement amidst the unfolding investigation.