Manchester, England (CNN) The UK raised its terrorism threat level to "critical" -- the highest level -- for the first time in a decade Tuesday.

Prime Minister Theresa May warned that intelligence services believe an attack may be "imminent."

Hours earlier, police identified 22-year-old Salman Abedi as the suspected suicide attacker who detonated a bomb as throngs of teenagers poured out of an Ariana Grande concert in Manchester, killing 22 people.

People hug near armed police who responded to the scene.

People hug near armed police who responded to the scene.

An injured man is helped at the scene.

An injured man is helped at the scene.

People sit outside near the arena. The crowd was made up of mainly younger people who had come to see Grande in her first of three scheduled concerts in the UK.

People sit outside near the arena. The crowd was made up of mainly younger people who had come to see Grande in her first of three scheduled concerts in the UK.

People are escorted away from the arena.

People are escorted away from the arena.

Greater Manchester Police tweeted that emergency services were "responding to (a) serious incident at Manchester Arena. Avoid the area. More details will follow as soon as available."

Greater Manchester Police tweeted that emergency services were "responding to (a) serious incident at Manchester Arena. Avoid the area. More details will follow as soon as available."

People receive medical attention at a railway station close to the arena. The incident happened shortly after Grande had left the stage, shortly after 10:30 p.m. (5:30 p.m. ET) according to eyewitnesses. Calvin Welsford, an 18-year-old concertgoer, said that he heard a "loud bang" a couple of minutes after Grande's set had finished.

People receive medical attention at a railway station close to the arena. The incident happened shortly after Grande had left the stage, shortly after 10:30 p.m. (5:30 p.m. ET) according to eyewitnesses. Calvin Welsford, an 18-year-old concertgoer, said that he heard a "loud bang" a couple of minutes after Grande's set had finished.

People gather outside the arena. "We can confirm there was an incident as people were leaving the Ariana Grande show last night," police said on Twitter early on Tuesday. "The incident took place outside the venue in a public space. Our thoughts and prayers go out to the victims."

People gather outside the arena. "We can confirm there was an incident as people were leaving the Ariana Grande show last night," police said on Twitter early on Tuesday. "The incident took place outside the venue in a public space. Our thoughts and prayers go out to the victims."

Police help someone after the attack at Manchester Arena on Monday, May 22.

Police help someone after the attack at Manchester Arena on Monday, May 22.

British Prime Minister Theresa May condemns the "callous terrorist attack" as she delivers a statement in London on May 23.

A forensics team works at the scene of the explosion on Tuesday, May 23.

A forensics team works at the scene of the explosion on Tuesday, May 23.

Women cry after placing flowers in Manchester on May 24.

Women cry after placing flowers in Manchester on May 24.

Police officers deliver flowers to a makeshift memorial in Manchester on Wednesday, May 24.

Police officers deliver flowers to a makeshift memorial in Manchester on Wednesday, May 24.

Local residents hold Manchester City and Manchester United soccer jerseys during the national minute of silence.

Local residents hold Manchester City and Manchester United soccer jerseys during the national minute of silence.

A Manchester road is closed off as police raids continued on May 25.

A Manchester road is closed off as police raids continued on May 25.

A women sheds tears after observing the minute of silence in St. Ann's Square.

A women sheds tears after observing the minute of silence in St. Ann's Square.

Flowers and tribute messages are left for victims in St. Ann's Square.

Flowers and tribute messages are left for victims in St. Ann's Square.

A police officer guards a house in Manchester as investigations continued on May 25. Police say a man carrying explosives acted as a lone attacker and died in the blast.

Britain's Queen Elizabeth II speaks to 15-year-old Millie Robson and her mother, Marie, during a visit to the Royal Manchester Children's Hospital on May 25. The Queen was visiting those injured in the attack.

People in Manchester, England, gather in St. Ann's Square on Thursday, May 25. They were observing a national minute of silence to remember the victims of a suicide bombing at an Ariana Grande concert.

Key developments

Police have carried out two raids in Manchester.

Queen Elizabeth II described bombing as "act of barbarity."

59 people injured, some in life-threatening situations.

US President Donald Trump slams attackers as "losers."

Ariana Grande suspends her world tour.

Lone attacker?

The suspect apparently acted as a lone attacker and died in the blast Monday night, police said, which left the wounded and the dead scattered across the arena's bloodied entrance and sent screaming girls running for cover.

The bombing has drawn condemnation and horror from around the world as a heinous assault targeting children.

Teen concertgoer Olivia Campbell, whose mother spoke to CNN during an agonizing wait for news from her daughter, is the latest victim to have been confirmed dead.

Fifteen-year-old Olivia had gone to the concert with her friend Adam to celebrate his birthday. She is one of four victims, including eight-year-old Saffie Rose Roussos , who have been identified.

Olivia Campbell, 15, had been confirmed as one of the dead.

While police believe Abedi acted alone, May warned that investigators could not discount the possible involvement of a "wider group" of terrorists.

The raising of the terror threat level means "not only that another attack remains highly likely but that a further attack may be imminent," she said.

It will enable the government to deploy armed military personnel of support of police and other security forces, particularly in the guarding of key sites, concerts and sporting events, May said.

Emergency crews evacuated victims at the Manchester Arena Monday night.

"I do not want the public to feel unduly alarmed," she said, calling the response "proportionate and sensible" to the threat level.

The blast marked the deadliest terror attack on British soil since the 2005 London bombings.

Claim of responsibility

ISIS claimed responsibility for the attack but offered no evidence. The terror group said on its Telegram channel Tuesday that a "soldier of the caliphate" was able to "plant explosive devices" at the arena, a US counterterrorism source told CNN.

ISIS routinely claims attacks it has no proven connection to.

Authorities have discovered no evidence of a link between the attacker and an established terror group, a British counterterrorism official told CNN.

A US counterterrorism official said bombing "looks much like" an ISIS attack but that American intelligence officials were working with British counterparts to determine more.

No determination has been made on the sophistication of the explosive device or what chemicals were involved, the official said.

Remembering the victims

"We will stand together to say that this city is greater than the force that aligns itself against it," David Walker, Bishop of Manchester, told the crowd.

"We are sending a signal not just to Manchester, but across the world that you can not defeat us because love in the end is always stronger than hate."

Saffie Rose Roussos, 8, was among those killed in the Manchester Arena attack.

Eight-year-old Saffie Rose Roussos from Leyland has been named as one of the fatalities, the Lancashire County Council confirmed.

Chris Upton, the head teacher at the Tarleton Community Primary School, described her as "simply a beautiful little girl" who was "quiet and unassuming with a creative flair."

Georgina Callander, 18, was also killed, according to her school, the Bishop Rawstorne Church of England Academy.

"All of our students will gather together today for a time of prayer and reflection and to give thanks for the life of Georgina," the school said.

At least 12 victims aged 16 or under were being treated at a children's hospital for serious injuries, some of them fighting for their lives, a Manchester health official said.

Grande, who had just finished the first of three scheduled UK performances, tweeted about her devastation several hours later: "broken. from the bottom of my heart, i am so so sorry. i don't have words."

broken.

from the bottom of my heart, i am so so sorry. i don't have words. — Ariana Grande (@ArianaGrande) May 23, 2017

The pop star has suspended her "Dangerous Woman" tour following the attack, a source with knowledge of the situation told CNN. Grande was scheduled to perform in London and across Europe through mid-June.

Queen Elizabeth II said Tuesday that "the whole nation has been shocked."

Around 400 police were deployed overnight following the attack, and on Tuesday, large groups of armed police were seen in several parts of the city. Security was boosted in London.

A poster from an Ariana Grande fan sits alongside flowers at Manchester's St Ann's Square. #manchesterattack A post shared by Bryony Jones (@bryonysjones) on May 23, 2017 at 7:40am PDT

Trump calls attackers 'losers'

JUST WATCHED Pres. Trump: Manchester attacked by losers Replay More Videos ... MUST WATCH Pres. Trump: Manchester attacked by losers 02:05

US President Donald Trump slammed the attack, saying that terrorists were "losers."

"So many young beautiful innocent people living and enjoying their lives murdered by evil losers in life. I won't call them monsters because they would like that term. They would think that's a great name. I will call them from now on losers because that's what they are," he said. "This wicked ideology must be obliterated."

The US Department of Homeland Security said it was "closely monitoring" the situation.