LONDON (AP) — 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 has previously said there are strong indications Iran was linked to the apparent plan to attack RAF Fairford last month. Seven men have been arrested and released on bail as counterterror police investigate.

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.”