The so-called Islamic State terrorist group has claimed responsibility for Monday's blast at a concert by U.S. pop star Ariana Grande in Manchester, England that killed at least 22 people and injured dozens more.
The group said "a soldier of the caliphate" was responsible for the attack on people IS described as "crusaders." Many of those killed and injured in the blast were children and teenagers, police said.
The top U.S. intelligence official, though, said it is not yet clear whether the Islamic State group was behind the attack.
Director of National Intelligence Dan Coats told Congress the extremist group frequently claims responsibility for terror attacks.
British police said investigators believe the attacker was carrying an improvised explosive device, which he detonated, and that he died at the site. The blast injured at least 59 people.
Prime Minister Theresa May said police and security officials believe they know the identity of the attacker, but are not yet ready to confirm it publicly.
The police department later said on Twitter that officers had arrested a 23-year-old man in South Manchester in connection with the attack, but did not give any information about how he was involved.
The blast happened in the lobby of the 21,000 seat Manchester Arena at the end of a concert by Grande.
WATCH: Henry Ridgwell on latest developments
At a Glance: What We Know So Far
- At least 22 people were killed and 59 wounded in explosion Monday night. The blast happened in lobby of 21,000 seat Manchester Arena.
- Islamic State claims "a soldier of the caliphate" was responsible for attack, although U.S. intelligence officials have yet to confirm claim.
- British police said investigators believe attacker was carrying improvised explosive device, which he detonated, and that he died at site.
- Prime Minister Theresa May said police and security officials believe they know identity of attacker.
- -British police said they arrested a 23-year-old man in South Manchester in connection with attack, but did not give any information about how he was involved.
Reaction from Grande
"Broken. From the bottom of my heart, I am so, so sorry," Grande wrote on Twitter after the blast. "I don't have words."
May said the blast was timed to "cause maximum carnage" and targeted "the young people of our society with cold calculation."
"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," she said.
May and Jeremy Corbyn, the leader of the opposition Labour Party, agreed to suspend campaigning ahead of the country's June 8 elections.
US monitoring situation
The U.S. Department of Homeland Security said it was monitoring the situation in Manchester, and that it did not have any information showing a "specific credible threat" to music venues in the U.S.
President Donald Trump said the victims in Wednesday's attack were killed 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," Trump said. "I will call them from now on losers, because that's what they are."
He added, "We cannot stand a moment longer for the slaughter of innocent people."
WATCH: Trump reacts to Manchester attack
After the attack, Manchester police deployed hundreds of officers overnight and at one point conducted a precautionary controlled explosion near the arena of an object they later said turned out to not be anything suspicious.
Video from the concert showed thousands of concertgoers, many of them young girls, scrambling and screaming, trying to escape the building.
Some witnesses said the ground near the blast was covered with nuts and bolts.
Abandoned shoes, phones and jackets were scattered throughout the arena.
"It was a huge explosion. You could feel it in your chest. It was chaotic. Everybody was running and screaming just trying to get out," a concertgoer told Reuters.
In Photos: Manchester suicide bombing
Search for survivors, victims
Worried parents who had brought their children to the show crowded the streets outside the building. A nearby hotel opened its doors to the kids looking for their mothers and fathers.
Cab drivers turned off their meters and offered to drive people from the ill-fated concert to wherever they want to go.
A vigil will be held Tuesday night at 6 p.m. near the scene of the attack. While the area around Manchester Arena is still blocked off, Greater Manchester Police said they want to remind people that "Manchester will not be defeated."