British authorities late on May 23 deployed the army to protect against another possibly "imminent" attack as they searched for accomplices to the suicide bomber who killed 22 people and injured 59 in a Manchester concert hall.
Prime Minister Theresa May said it was possible that a wider group was linked to the bombing, which hit the mostly young crowd exiting after a performance by pop star Ariana Grande, prompting authorities to raise the alert level to the highest possible "critical" stage.
"This means that their assessment is not only that an attack remains highly likely, but that a further attack may be imminent," May said in a televised statement from her Downing Street Office even as thousands poured into the streets of England to mourn the victims.
"Armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces... You might also see military personnel deployed at certain events, such as concerts and sports matches."
Manchester Police Chief Ian Hopkins identified the bomber as Salman Abedi, 22, who authorities said died in the attack. May said Abedi was born and raised in Britain and security officials said he was of Libyan descent.
Police raided two sites in the northern English city, setting off a controlled explosion in one, and arresting a 23-year-old man in a third location.
The extremist group Islamic State claimed responsibility for the attack on what it called "crusaders" on its Telegram account, and warned that"what comes next will be more severe on the worshippers of the cross."
IS's Amaq news agency spoke of "a group of attackers" in an online post that was later removed.
Under Britain's Operation Temperer, army troops may be moved in to replace police officers who now guard "key sites" such as sports stadiums and major entertainment venues.
May said the move will allow the police to significantly increase the number of armed officers on patrol.
May took calls throughout the day on May 23 from many world leaders, who expressed outrage at the attack and solidarity with Britain, which has not seen such a deadly attack since 52 people were killed by suicide bombings on London's metro system in 2005.
The attack also elicited painful memories of the 2015 terror attacks in Paris, where most of the 130 killed were at the Bataclan concert hall.
"We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish but as an opportunity for carnage," May said in her address to the nation.