LONDON, Oct 1 (Reuters) - British counter-terrorism police investigating a suspected plot involving an air base used by the US to target Iran said on Thursday they had arrested a dual UK-Iranian national on suspicion of preparing terrorist acts.
In an incident which triggered a huge police response, raised alarm among Western allies, and remains largely unexplained, five British men in their 20s were arrested on Sunday on suspected terrorism and explosive offences near RAF Fairford in western England.
The men were later released without charge on police bail. But counter-terrorism police said they were considering whether a foreign state had been involved in the incident and Prime Minister Andy Burnham said on Wednesday it might have had an Iranian link.
Iran has said accusations it was involved were baseless and on Thursday summoned the British ambassador in Tehran over the issue.
In the latest development, police said they had arrested the 27-year-old dual national in London while a second British man, 26, had also been questioned under caution and searches carried out at two properties.
"The investigation into circumstances surrounding events in Gloucestershire is hugely complex, and our specialist teams are interrogating multiple lines of enquiry," Britain's Senior National Coordinator for Counter Terrorism Policing, Vicki Evans, said in a statement.
"As we’ve made clear, we’re looking at all possible angles – including possible foreign state involvement."
Police launched a massive operation in the early hours of Sunday "due to a range of information" including an alert from a local farmer who became suspicious of the men who she had seen near the base in three vans.
However, the Times newspaper reported that one of the men had also called emergency services shortly before being arrested.
Detectives have confirmed that although petrol was found in the vans used by the men, no improvised explosive device was present.
Burnham said Britain was working closely with the US, whose bombers have in recent months used Fairford to launch missions against Iran, although US President Donald Trump said he would not have released the men, who police said were only freed under stringent conditions.
Trump previously said those involved had planned "big damage" while US Secretary of State Marco Rubio also pointed to the "hands of a foreign actor" in the incident.
(Reporting by Sam Tabahriti and Michael Holden; editing by Philippa Fletcher and Andrew Heavens)
