MANCHESTER, England >> Britain began mobilizing troops to help protect key locations Tuesday as the government warned of the potential for another terrorist attack in the wake of a suicide bombing that killed 22 people outside a Manchester arena.

For the first time in 10 years, the government raised its terrorism threat level from “severe” to “critical” after the militant group Islamic State claimed responsibility for the Monday night attack that also injured at least 59 people leaving an Ariana Grande concert.

“The work undertaken throughout the day has revealed that it is a possibility we cannot ignore that there is a wider group of individuals linked to this attack,” Prime Minister Theresa May announced, adding that counterterrorism officials believe “not only that an attack remains highly likely, but that a further attack may be imminent.”

The 22-year-old bomber, identified as Salman Abedi, is an English-born son of Libyan immigrants who was a student at the University of Salford in Manchester.

The attack is the latest to plunge Europe into a cycle of grief, fear and political recriminations, as Muslims in Manchester condemned the bombing, parents anguished over the many youngsters killed and injured, and authorities struggled to learn whether the bomber had help.

“So many young, beautiful, innocent people living and enjoying their lives murdered by evil losers in life,” President Donald Trump said while visiting Bethlehem in the West Bank. “I won’t call them monsters because they would like that term, they would think that’s a great name. I will call them losers because that’s what they are.”

In Manchester, thousands of well-wishers crowded into Albert Square for a Tuesday evening vigil to hear officials pay tribute to the dead beneath the Gothic spire of the Manchester Town Hall.

“Today is a day that we all hope and prayed we would not ever see. Families, young children went out last night to enjoy themselves in our wonderful city and tragically lost their lives in a horrific way,” Greater Manchester Police Chief Constable Ian Hopkins told the gathering.

Mohammed Shafiq, chief executive of the Ramadhan Foundation in Manchester, said the deadly explosion marked the “darkest day” in the city’s history.

“I love Manchester and its people. We are a resolute people and will not be divided by these barbaric animals or cowered by their violence,” he said.

Abedi had been investigated by British authorities for unspecified reasons, according to a U.S. law enforcement official who has been briefed on the investigation.

His 23-year-old brother has also been taken into custody, the official said, but it is unclear whether he will be charged.

In a statement released on social media, Islamic State said a “soldier of the (caliphate)” had “managed to place explosive devices in the midst of the gatherings of the Crusaders in … Manchester.”

It said the attack was an “endeavor to terrorize the mushrikin,” using the Arabic word for polytheists, a pejorative that includes Christians, according to the group’s ideology, and it threatened to carry out more attacks.

“What comes next,” it said, “will be more severe on the worshipers of the Cross and their allies, by Allah’s permission.”

Claims of previous lone-wolf attacks perpetrated by Islamic State supporters have often come from the Amaq news agency before being acknowledged by Islamic State in a later statement. But Tuesday’s claim first came from Islamic State’s media branch, hinting the attacker had a more direct link to the group.

Police said the blast was triggered by a man carrying an improvised explosive device near the exits at the arena just a few minutes after Grande had finished singing her final song, “Dangerous Woman,” and left the stage. Grande was not harmed and has said she was devastated by the attack.

The bomb used shrapnel designed to cut and kill, but was crudely designed and investigators are determining whether it was homemade, according to U.S. law enforcement officials briefed on the British investigation.

It is unclear to U.S. intelligence officials what role, if any, Islamic State may have played in the Manchester bombing, whatever its claims of responsibility, a U.S. official said.

The last time Britain’s threat level was set to critical, in 2007, came a day after officials discovered two car bombs near the Haymarket neighborhood of London. The devices were disabled before they could explode.

Two men who plotted to plant the car bombs went on to ram a vehicle into the international airport in Glasgow, Scotland, where one of them was captured and the other was killed.

In her announcement Tuesday, May said military personnel will be deployed at key locations around the country to support the police.

“This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces, which will allow the police to significantly increase the number of armed officers on patrol in key locations,” May said. “You might also see military personnel deployed at certain events, such as concerts and sports matches, helping the police to keep the public safe.”

An investigator in protective forensic gear was photographed carrying a booklet titled “Know Your Chemicals!” out of a Manchester address linked to Abedi as officials executed search warrants on two properties in the area to determine how the bomb was assembled and whether he had any help.

Abedi’s parents came to Britain after fleeing Moammar Gadhafi’s government in Libya and had three sons and a daughter, according to neighbors and news reports.

Abedi lived quietly with his brothers in the working-class Fallowfield area of Manchester after their parents moved back to Libya in 2011 following the fall of Gadhafi’s government. All of them kept to themselves, neighbors said.

“I would see him walking down the street, but he would not bother to say hello,” said one resident, who asked not to be identified. “The only problem I ever had — apart from the boys kicking a football against my back fence — was when his shed burnt down. It backs onto mine, so mine burnt down as well. I now dread to think what he was doing in there. I wonder now if he was making something like a bomb.”

Farag Elkailani, a 53-year-old car mechanic, said Abedi went to the local mosque five times a day and often led prayers. “He never looked as if he had been radicalized,” he said. “I never really thought he was that religious despite his prayers.”

One local imam said Abedi had looked at him with hatred after the imam gave a speech denouncing terrorism and Islamic State in 2015.

“I could understand that he was not happy with me because I did combat (Islamic State) in that Friday sermon sometimes,” Mohammed Saeed El-Saeiti told the Telegraph newspaper.

The youngest victim of the attack to be named so far is Saffie Rose Roussos, 8.

She was at the concert with her mother and older sister, who were injured and later found in separate hospitals, the Manchester Evening News reported.

“Saffie was simply a beautiful little girl in every aspect of the word,” her school principal, Chris Upton, said in a statement.

Manchester’s city center was in lockdown Tuesday.

Armed police walked the streets, traffic was at a standstill on roads close to the arena, and a police cordon kept passersby and drivers at least 150 yards from the point of the blast.

Coincidentally, the bomb appeared to have been detonated just around the corner from where the Irish Republican Army unleashed its biggest-ever bomb on the British mainland in 1996.

In that attack, people were given 90 minutes’ warning before 3,300 pounds of explosives loaded into a truck shattered the city; on Monday, there was no such warning.

On the main road behind the arena, a small crowd gathered to pay its respects and quietly contemplate the events.

Brian Crawford, 45, said he had been in the area Monday night, seconds after the explosion, but was unaware of the severity of what had taken place.

He had been to a yoga class and was walking to his car as the alarms at Victoria Railway Station went off, sending a deafening noise across the city.

“All I could hear was three long shrieks of the sirens and then an automated voice warning people something was wrong,” he said. “Then it would repeat. I could not even tell what the voice was saying, but I knew something was wrong.”

Emergency vehicles with sirens shrieking soon came roaring past. “I saw a row of about 10 ambulances going in, and I just got out as quick as I could.”

Campaigning for the June 8 parliamentary elections was suspended, and May urged the public to stand united in the days ahead.

“The terrorists will never win, and our values, our country and our way of life will always prevail,” she said.

Manchester Mayor Andy Burnham said the evening vigil was intended as a place for grieving members of the community to come together.

“For individuals to go there and seek to terrorize those children and those young people and their families in that way is the most appalling evil I can imagine,” he told reporters.

“The individual who carried this out is an extremist and doesn’t represent any of our communities, does not represent the people of Greater Manchester in any way, shape or form.”

Officials in Brussels said they would not be deploying additional security measures for Trump, who arrives Wednesday and attends a summit the following day at NATO headquarters.

“We have already taken the maximum security measures,” said Brussels Mayor Yvan Mayeur, noting that more than 4,000 police officers would be deployed in the Belgian capital during the summit.

Since the city was the target of deadly suicide bombings last year, he said, the possibility of an attack and ways to respond to it are “systematically considered.”

Newly elected French President Emmanuel Macron called May to offer his country’s sympathy and support.

“It is all of Europe, free Europe, that has been attacked,” he told reporters at the British Embassy in Paris, where he signed a book of condolences for the victims of the bombing.

Macron said he would convene a Defense Council meeting Wednesday to begin forming an anti-Islamic State task force, a key campaign promise.

“It is also our wish to reinforce European cooperation in the fight against terrorism,” he said.