WATCH: Theresa May Increases Terror Threat Level in United Kingdom to Critical

British Prime Minster Theresa May — in an address to the nation late Tuesday evening — increased her country’s terror threat level from severe to critical, saying “a further attack may be imminent.”

“It is a possibility we cannot ignore that there is a wider group of individuals linked [to the Manchester attack],” May said.

The “critical” classification is the country’s highest terror threat level.

On Tuesday morning, May delivered a speech outside Downing Street — condemning the “sickening cowardice” of the Manchester attacker, Salman Abedi.

WATCH her full statement below:

At least 22 people were killed — including an 8-year-old girl — and 59 others were injured after a suicide bomber detonated himself outside the Manchester Arena just as a concert for pop singer Ariana Grande was concluding on Monday night.

“All acts of terrorism are cowardly attacks on innocent people, but this attack stands out for its appalling, sickening cowardice, deliberately targeting innocent, defenseless children and young people who should have been enjoying one of the most memorable nights of their lives,” May said on Tuesday morning.

“We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish but an opportunity for carnage.”

General election campaigning has been suspended due to Monday’s incident and May will chair a meeting of the government’s emergency Cobra (Cabinet Office Briefing Room) committee on Tuesday morning.

Labour Party leader Jeremy Corbyn reacted to the incident via Twitter:

Liberal Democrats leader Tim Farron also issued a statement:

President Donald Trump — who was in Bethlehem as part of his first foreign trip as U.S. president — called those responsible for the attack “evil losers.”

The Islamic State claimed responsibility for the attack, writing in a statement, a “caliphate soldier managed to place a number of devices among a gathering of crusaders in Manchester, and detonated them.” It should be noted that there was just one explosion in the attack, and no other ‘devices’ were found at or near the scene.

U.S. intelligence sources told CBS News they were exercising caution on the early claim of responsibility from ISIS, and were looking into the possibility Abedi acted alone.

A representative for the singer says she is OK and they are investigating what happened.