Britain Raises Threat Level to Highest and Warns New Attack May Be Imminent

British Prime Minister Theresa May said Tuesday night the U.K. raised its threat level to critical, the highest level possible, indicating more attacks might be imminent.

The announcement came the day after a suicide bomb attack that killed 22 people at an Ariana Grande concert in Manchester.

The prime minister also said the government has triggered "Operation Temperer," which is an emergency plan that places soldiers at key public locations to support police in protecting the public, the BBC reported.

"As we mourn the victims of last night's attack, we stand defiant," May said, according to a CBS report, adding that this is the highest the threat level has reached in the U.K. in 10 years. "The spirit of Manchester and the spirit of Britain is far mightier than the sick plots of terrorists—and that is why the terrorist will never prevail."

May met with her top security officials after the Manchester attack and said afterward that, "It is a possibility we cannot ignore that there is a wider group of individuals linked to this attack," according to The New York Times.

The terrorism threat level is decided by the Joint Terrorism Analysis Centre, which consists of experts from the police, governmental departments and agencies.

The Islamic State militant group (ISIS) claimed responsibility for the attack in a statement posted on Twitter.

"One of the soldiers of the Caliphate was able to place an explosive device within a gathering of the Crusaders in the city of Manchester," the statement said. ISIS refers to those who commit attacks in its name, inspired or directed by the group, as "soldiers."

It's not clear whether the group directed the attacker, as it did with the assailants who killed 130 people across Paris in a series of attacks in November 2015.