Story highlights Soldiers will replace police officers at key sites

They also will be deployed at concerts and sporting events

(CNN) The United Kingdom will deploy soldiers to take over guard duties at key sites so elite police can focus on the threat of another terror attack.

While the sight of a member of the armed forces outside concert venues and sporting events may be jarring, in every case the soldiers will be under the command of police officers, the Prime Minister said.

The presence of troops comes as the UK raised its threat level to "critical," its highest. Such a level indicates another attack could be "imminent."

Authorities have been trying all day to determine whether Salman Abedi, the 22-year-old British-born man who they say carried out the attack outside Manchester Arena on Monday night, acted alone.

They haven't ruled out the possibility that others might be involved.