The United Kingdom has set its terrorism threat level as Critical—the highest possible level. It means terrorist attacks might be imminent.
Soldiers have been deployed at key points like Buckingham Palace and Parliament.
U.K. police continue to patrol Manchester.
Police said Salman Abedi, 22, was the suicide bomber who struck the Ariana Grande concert.
Abedi was a British-born Libyan. He made several trips back to Libya.
Home Secretary Amber Rudd said Abedi likely did not act alone.
Four others have been arrested in connection with the attack.
Manchester residents have been laying wreaths and notes of sorrow and support at makeshift shrines.
