Hundreds of British Army soldiers were ordered to fan out throughout the U.K. on Wednesday to help police protect “key sites” in the aftermath of Monday night’s terror attack at an Ariana Grande concert in Manchester. The deployment came hours after Prime Minister Theresa May’s government raised the U.K. terror threat level to “critical,” meaning officials believe more attacks could be imminent. As a precaution, Westminster Palace has been closed to the public. Worries about additional strikes heightened as Home Secretary Amber Rudd indicated that investigators believe Salman Abedi, who has been identified as the suicide bomber that killed 22 people—many of them young girls—was “likely” to have not acted alone. “It was more sophisticated than some of the attacks we’ve seen before, and it seems likely—possible—that he wasn’t doing this on his own,” Rudd said. On Tuesday, authorities identified Abedi and arrested his brother, Ismail Abedi. The Islamic State terror group claimed responsibility for the attack, but analysts have yet to confirm that involvement. Separately on Wednesday, three more men were arrested in South Manchester in what is said to be connection with the attack. The Greater Manchester Police also announced that coroners have identified all of the dead and are in contact with families.