While British police have identified the suicide bomber, who killed 22 and injured dozens after an Ariana Grande concert in Manchester, as 22-year-old Salman Abedi, they’re trying to determine if he acted alone.

Prime Minister Theresa May says the investigation’s revealed it’s possible there was a ”wider group of individuals linked to this attack.”

As a result experts feel the threat level should be increased for the time being from ”severe” to ”critical”.

”This means that their assessment is not only, that an attack remains highly likely, but that a further attack may be imminent.”

Members of the British Armed Forces will boost security at key sites and military personnel might be deployed at public gatherings such as concerts and sporting events.

British police don’t routinely carry firearms, but London police said extra armed officers would be deployed at this weekend’s soccer cup final at Wembley and rugby at Twickenham.  

Thousands attended a vigil for the victims.

(Kristy Wigglesworth / The Assoicated Press)

The Manchester attack was the deadliest in the U.K. since four British Muslims killed 52 in suicide bombings on London’s transport system in 2005. 

However, in March, a British-born convert to Islam drove a car into pedestrians on London’s Westminister Bridge, killing 4 before stabbing to death a police officer on the grounds of Parliament.

The attacker was shot and killed by police.