Updated 12:20 p.m. EDT, May 23, 2017

LONDON—Twenty-one thousand concertgoers—many of them teenagers—crowded into the Manchester Arena to see Ariana Grande on Monday night. It was the second stop on the British leg of her Dangerous Woman Tour.

The real threat was lurking just outside.

Minutes after the former child star—now 23—finished her encore and fans began to make their way outside, a lone attacker detonated an improvised explosive device at the entrance of the venue in Manchester, England.  

At least 22 people were killed and 59 injured, Chief Constable Ian Hopkins of the Greater Manchester Police said. “I can confirm there are children among the deceased,” he said.

The first victims to be identified were Saffie-Rose Roussos, who was just 8, and Georgina Callandar, 18, who was described by her former school as “a lovely young student.”

Callender, an Ariana Grande superfan, met her hero on a previous tour of the U.K. before posting a picture of them hugging on Instagram.

Authorities believe the attack was carried out by one man, who they have identified. The British authorities confirmed that the suspect is named Salman Abedi, 22. He died at the scene.

Whether he acted alone or is connected to a network is the focus of the investigation now. The Islamic State claimed responsibility for the attack through official and semi-official channels on Tuesday morning.

A senior U.S. counterterorrism official says the have "no reason to doubt the claims" of responsibility for the attack, but offered no clarity on whether ISIS directed it or simply inspired the attack. The official spoke on condition of anonymity to discuss the still evolving investigation.

Security officials say they believe around 350 jihadis have returned to Britain from the conflict in Syria. The possibility that the bomb maker is still at large is a particular concern; this is the first time terrorists have successfully used explosives for an attack on Britain since July 7, 2005, when four suicide bombers blew themselves up on London public transport, killing 52 people.

Manchester police said they had arrested a man, 23, on in connection with the attack.

“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,” said Prime Minister Theresa May.

U.S. President Donald Trump expressed his condolences and launched into a vehement denunciation. “I won’t call them monsters because they would like that term,” he said. “I will call them from now on ‘losers.’”

A senior official at the U.S. Department of Homeland Security told The Daily Beast that U.S. officials were working off the understanding that there was a person who was trying to get into the concert area and that he was stopped by a security guard.

The crowd—many of whom were children and teenagers—scrambled to escape the arena after the attack, falling over each other and sustaining injuries in the panic as more than 20 bodies lay prone, dead, or injured in the foyer.

One father, who was there to pick up his wife and daughter, described being blown off his feet by the force of the blast.

Peter Byrne/AP

“We got thrown forward through the door toward the arena—the explosion was behind us forcing us forward,” Andy Holey told the BBC. “Everyone that was 10-15 feet behind us; every one of them got injured. There was about 20-30 people fatally injured.”

“I could hear the singer on her last song and then as it finished people were coming out, and then I just heard this loud bang, felt a lot of stuff near me ears and it threw me forward. When I got up and looked behind me there was just people lying everywhere so I just ran into the stadium to see if I could see my family, but couldn’t find them.”

After a frantic search inside the arena, where thousands of concert goers were fleeing the hall amid giant pink balloons, Holey raced back out to the box-office area.

“I started checking people that were lying there... I don’t know if they was dead or unconscious but I had to check in case my family were there,” he said. “Just felt sick to my stomach, because every time I looked at a person—as bad as it was for that person—I was praying it weren’t me wife or daughter.”

Holey eventually found his family safe and well, and they sought shelter at a nearby hotel, where dozens of people were in the lobby well after midnight watching news of the atrocity as it filtered through on television screens.

Goodman/LNP/REX/Shutterstock/AP

The timing of the attack comes shortly before Britain’s upcoming general election, which will be held in just over two weeks. Prime Minister May has suspended her campaign as security agencies hunt down the attacker.

“It is now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack, an attack that targeted some of the youngest people in our society with cold calculation,” she said.

“This was among the worst terrorist incidents we have ever experienced in the United Kingdom. And although it is not the first time Manchester has suffered in this way, it is the worst attack the city has experienced and the worst-ever to hit the north of England.”

Manchester’s city center was attacked by the Provisional IRA in 1996 with the biggest bomb to be detonated in Britain since the Second World War. The Arndale shopping mall was destroyed and 212 people were injured although there were no fatalities. It took a decade for Britain’s second city—which is one of the cultural hearts of the nation—to be rebuilt.

While ISIS supporters celebrated the bloodshed on social media, unconfirmed reports on social media include claims of a now-suspended Twitter account that posted about the attack shortly before it occurred, and a video threatening Manchester posted to Telegram. The Daily Beast was unable to independently verify those reports.

In the latest issue of the ISIS magazine Rumiyah, however, the terrorist group listed concert halls—along with nightclubs, movie theaters, shopping centers, and swimming pools—as possible targets.

Andrew Yates/Reuters

“Generally any busy enclosed area, as such an environment allows for one to take control of the situation by rounding up the kuffar [unbelievers] present inside and allows one to massacre them while using the building as a natural defense against any responding force attempting to enter and bring the operation to a quick halt,” the magazine noted. It also advised targeting dark areas to further hamper first responders.

ISIS wrought havoc with similar tactics less than two years ago on the Bataclan concert hall, where 130 people were killed during an Eagles of Death Metal show in Paris. A statement in the aftermath from ISIS hailed the attack, describing the scene at the Bataclan as a venue where “hundreds of apostates had gathered in a profligate prostitution party.”

Years earlier, in 2003, a pair of female suicide bombers attacked a concert outside Moscow, killing 14 people.

Mia Bloom, a Georgia State University professor and author of several books on terrorism, told The Daily Beast that jihadis have convinced themselves that teenagers out partying should be considered legitimate targets.

Reuters

“There’s a reason rock concerts are so appealing: You get young people, you get people in a crowded space... And generally, you’re not going to get what they [jihadis] consider a to be a good, religious, Muslim girl. There’s a selection effect,” Bloom said, while noting that no terrorist group has claimed credit for the attack.

“Plus, concerts are symbolic of the licentiousness and degradation of the West. There’s a reason why the first time [extremists] move into an area, they ban music and dancing. It’s like that Footloose thing,” Bloom added.

Boston rapper BIA, one of the opening acts at the Manchester Arena, tweeted and then deleted a message that read “GUYS WE ARE OKAY!!” It was replaced with with words of mourning: “My heart is broken.”

Ariana Grande, who has since reportedly canceled two scheduled shows in the U.K. later this week, was not injured in the blast. In a statement released on Twitter, Grande said she felt “broken.”

Scooter Braun, Grande’s manager, issued a statement on social media echoing the singer’s sentiments.

“Tonight, our hearts are broken,” Braun said. “Words cannot express our sorry for the victims and families harmed in this senseless attack. We mourn the lives of children and loved ones taken by this cowardly act. We are thankful for the selfless service tonight of Manchester’s first responders who rushed towards danger to help save lives. We ask all of you to hold the victims, their families, and all those affected in your hearts and prayers.”

Concertgoer Hannah Dane told The Guardian there was “quite a loud explosion heard from inside the Manchester arena and it shook, then everyone screamed and tried to get out.”

Another witness, 26-year-old Suzy Mitchell, said a huge bang rocked the neighborhood, where she lives. Mitchell told the Press Association: “[I] just heard a huge bang from my bed, came out to the front of my apartments (we’re on the top floor so have perfect view) and everyone was running away in big crowds.”

“The bang was so big I heard it from my room which is at the back of the apartment blocks.”

Robert Gates, retired secretary of defense and former CIA chief, said the attack bore the hallmarks of ISIS.

"The speed with which ISIS took credit... lends credibility to them being responsible for it," he said. "Whether they planned it, whether they encouraged some radicalized person or group to do it is kind of immaterial as long as they were the spark."

Gates warned that as coalition forces drive ISIS out of Raqqah and Mosul, that the fleeing fighters will return to their west, still determined to kill. Speaking at a gathering of Washington, D.C., think tank Foundation for Defense of Democracies, Gates predicted that ISIS would metastasize, just as Al Qaeda had after the killing of Osama bin Laden.  

"Sadly, Manchester may be a harbinger," he said.

— with additional reporting by Noah Shachtman and Christopher Dickey