The U.K. has increased its terrorist threat level to the highest possible “critical” for the first time in a decade, British Prime Minister Theresa May announced Tuesday. The designation means a terror attack is considered “imminent” and allows for military personnel to be deployed instead of police officers at public events. The decision comes after a lone male suicide bomber detonated an explosive Monday night near the Manchester Arena, where the singer Ariana Grande was performing. At least 22 people were killed and 59 injured in the blast. Police identified 22-year-old Salman Abedi as the bomber, and the Islamic State claimed responsibility for the attack over social media. Police on Tuesday arrested a 23-year-old man in connection with the attack.