British counterterrorism police have arrested a dual British-Iranian citizen as investigators probe an alleged terrorist plot targeting RAF Fairford, the strategically important British airbase used by the U.S. military for operations against Iran. The arrest comes after British Prime Minister Andy Burnham said there are “strong indications” that Iran played a role in the incident.

The 27-year-old suspect was taken into custody Thursday afternoon in Westminster, London, on suspicion of preparing acts of terrorism. His arrest is the sixth made in connection with the investigation, which began after five men were detained near RAF Fairford in Gloucestershire early Sunday morning.

Police also questioned a 26-year-old British citizen under caution as part of the expanding investigation, although he was not arrested. Counterterrorism officers searched two properties in Westminster as investigators continued attempting to establish whether the alleged plot had connections to a foreign government.

The original five suspects, British men in their 20s from London, were arrested after police received a report shortly before 1 a.m. Sunday concerning three suspicious vans near the military installation. A local resident had spotted unusual activity involving the vehicles and men near the base and contacted authorities.

Armed police descended on the area and detained the five men, initially under Britain’s Explosives Act and subsequently on suspicion of preparing an act of terrorism. A massive security operation followed, with approximately 85 households evacuated, a 400-meter cordon established and military bomb-disposal specialists brought in to examine the vans.

Although initial reports raised fears that the vehicles contained explosive devices, subsequent examinations found no explosives. Petrol was discovered inside the vans, however, and authorities are continuing to investigate what the men intended to do with the vehicles and their contents. The five original suspects were released on police bail Monday while remaining under investigation.

The possible Iranian connection has become an increasingly significant focus of the investigation following the arrest of the dual British-Iranian citizen.

Senior national coordinator for counter terrorism policing for the Metropolitan Police Vicki Evan stated: “We’re looking at all possible angles – including possible foreign state involvement.”

British authorities have stressed that the investigation remains open and that investigators have not publicly established precisely what role, if any, the Iranian government or its agents may have played.

Burnham, however, went considerably further Wednesday, publicly revealing that British authorities have information pointing toward possible Iranian involvement.

“There are strong indications that Iran played a part in what happened over the weekend at RAF Fairford, but of course it remains an ongoing, complex, and serious police investigation,” Burnham told BBC News. “And of course, we are working closely with our colleagues in the United States of America. So it’s ongoing.”

The prime minister said he convened the government’s Cobra emergency committee while he was in Liverpool and stressed that British authorities were treating the case at the highest levels.

“We are on top of it in every possible way imaginable,” he said.

“We will say more in due course, but we can confirm our belief that we believe Iran to have played a part,” he added. Asked why he believed Iran was involved, Burnham said, “Well I can’t go into all of those details, but obviously I wouldn’t say it lightly.”

The case has also drawn direct attention from Washington because of RAF Fairford’s role as a major operating location for the United States Air Force. American bombers based at Fairford have been used in operations connected to the conflict with Iran, making the installation an especially sensitive potential target.

President Donald Trump said following the original arrests that British and American authorities had been working together and that the suspects had been seeking to cause “big damage.” The president also indicated that U.S. authorities had been involved in investigating the group.

The Iran angle is particularly significant because Tehran had previously warned that military installations being used for attacks against Iranian territory could be regarded as targets. In July, Iran’s Islamic Revolutionary Guard Corps specifically drew attention to RAF Fairford’s use by American bombers and warned Britain over bases used in operations against Iran.

Iran has strongly denied involvement in the Fairford incident. Tehran summoned British Ambassador Hugo Shorter after Burnham’s remarks, with Iranian officials denouncing the British accusations and criticizing Britain for allowing its territory and military facilities to be used for American and Israeli operations against Iran.

The arrest of the dual British-Iranian suspect nevertheless adds a significant new dimension to an investigation that initially centered on the five men apprehended in the Gloucestershire countryside.

Those five suspects, all London residents in their 20s, were detained after a woman driving home spotted the suspicious vehicles and men near Whelford, close to RAF Fairford. The report triggered the rapid deployment of armed police and bomb-disposal teams and led to the evacuation of dozens of nearby homes.

Counterterrorism investigators are now working with American authorities to determine exactly what was being planned, whether the suspects were acting on behalf of anyone else, and whether the alleged operation was connected to Iran. For now, British officials have publicly pointed to evidence suggesting an Iranian role, while police continue to describe foreign-state involvement as one of several lines of investigation.