LONDON (AP) — British police have arrested a dual U.K.-Iranian national on suspicion of plotting a terror attack following a security incident at a a U.S.-run military air base in England.

The 27-year-old man was arrested in London on Thursday, and police have searched two properties in the U.K. capital.

A 26-year-old British national was interviewed by police under caution but was not arrested, Counter Terrorism Policing said in a statement.

On Sunday, five men, all in their 20s and from London, were arrested over the incident at RAF Fairford, which has been used by American bombers to strike Iran. They have since been released on bail.

Vicki Evans, the senior national coordinator for counter terrorism policing, said the investigation “is hugely complex, and our specialist teams are interrogating multiple lines of enquiry.

“As we’ve made clear, we’re looking at all possible angles — including possible foreign state involvement,” she said.

Prime Minister Andy Burnham has said the U.K. sees “strong indications” that Iran played a part in the incident. Iran denies involvement.