British Prime Minister Andy Burnham said Monday a security incident at a U.S.-run military air base in England was “almost certainly” directed by Iran’s Revolutionary Guard.

Burnham previously said there are strong indications Iran was linked to the apparent plan last month to attack the RAF Fairford air base, which has been used by American bombers to strike Iran in the ongoing war.

Police have arrested seven men in the investigation into the alleged plot, but all suspects have been released on bail and no one has been charged so far.

“I can confirm our assessment that it is highly likely to have been coordinated in Iran, almost certainly directed by Iran’s Islamic Revolutionary Guard,” Burnham told lawmakers. “We will not tolerate these threats on our soil.”

Scores of residents near the air base were evacuated from their homes on Sept. 27 as armed police officers and army bomb-disposal experts swarmed the rural area.

Police later said they had found a quantity of an accelerant — gasoline — in three vans linked to the initial five suspects, but no explosives.

All the suspects arrested are in their 20s and six are British nationals while one is a dual U.K.-Iranian citizen.

On Monday, the British government summoned the Iranian ambassador to express displeasure at “Iran’s continued attempts to undermine the security of the United Kingdom and international partners through its criminal networks, proxy groups and state-backed malign activity.”

The U.S. pulled its bombers from RAF Fairford last week, and the Trump administration has said it believed the base faced Iranian threats intended to kill as many American troops and destroy as many U.S. planes as possible.