British authorities have identified the suicide bomber as 23-year-old Salman Abedi.
Also Tuesday, Greater Manchester Police said they have arrested a 23-year-old man in connection with the apparent suicide bombing at an Ariana Grande concert. Police said the man was arrested in south Manchester Tuesday. Authorities did not provide details.
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Screams filled the arena just after the explosion, which also killed the attacker - a man carrying an IED that he detonated. Teens and children were among the victims.
British authorities said an 8-year-old girl, Saffie Roussos, was among dead. Twelve children under the age of 16 were among those injured.
WHAT WE KNOW:
- Bomber identified
- Islamic State claims responsibility
- 22 killed
- 59 injured
- single attacker died at the scene
- 23-year-old man in Manchester arrested in relation to attack
- 400 officers deployed
Police also said officials arrested a man at the Arndale shopping center in central Manchester - but that the arrest is not believed to be connected to Monday night's attack.
British Prime Minister Theresa May said Tuesday morning that police and security staff in Manchester believe they know identity of the apparent suicide bomber, but they are not revealing the name for the time being.
Speaking in London, May said: "This attack stands out for its appalling, sickening cowardice."
She says the attack, in which 22 people died, was one of the worst the nation had suffered.
The attack sparked a nightlong search for loved ones - parents for the children they had accompanied or agreed to pick up, and friends for each other after groups were scattered by the blast. Twitter and Facebook were filled with appeals for the missing.
Public transport shut down, and taxis offered to give stranded people free rides home, while residents opened their homes to provide lodging.
The concert was attended by thousands of young music fans in northern England. Grande, who was not injured, tweeted hours later: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
broken.— Ariana Grande (@ArianaGrande) May 23, 2017
from the bottom of my heart, i am so so sorry. i don't have words.
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President Donald Trump also expressed solidarity with the United Kingdom Tuesday after a meeting in Bethlehem, condemning the "evil losers" behind the blast.
RELATED: Trump calls out 'evil losers' behind deadly Manchester blast
Trump said the attack preyed on "innocent children." He said this "wicked ideology must be obliterated. And I mean completely obliterated." Adding, "civilized nations must join together to protect human life."
RELATED: Stars react to reports of explosion at Ariana Grande concert
Forensic investigations are trying to determine if the attacker had accomplices, said Chief Constable Ian Hopkins. He provided no information about the person who set off the bomb. Hopkins said police are treating the blast as an act of terrorism "until we know otherwise." The local ambulance service says 59 people were taken to hospitals.
Hopkins said some of the dead were children but provided no further details.
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The explosion struck just after 10:30 p.m. Monday night as Grande was ending the concert, part of her Dangerous Woman Tour. Police cars, bomb-disposal units and 60 ambulances raced to the scene as the scale of the carnage became clear. More than 400 officers were deployed. ABC News reports there were 240 calls to 911.
Manchester Arena said on its website that the blast struck outside the venue as people were leaving. Some eyewitnesses said it happened in the foyer.
"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena," said 22-year-old concertgoer Majid Khan. "It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit."
A message from The Duke of Cambridge following last night's attack in Manchester. pic.twitter.com/g8EWtuC2A8— Kensington Palace (@KensingtonRoyal) May 23, 2017
Home Secretary Amber Rudd decried "a barbaric attack, deliberately targeting some of the most vulnerable in our society - young people and children out at a pop concert."
The city's regional government and its mayor, Andy Burnham, were among scores of Twitter users who circulated the MissinginManchester hashtag, used by people looking for family members and friends.
Facebook has activated a safety check for attendees of the event. Click here to view the Facebook page.
Among the names being circulated was Olivia Campbell. Her mother, Charlotte Campbell, said the 15-year-old attended the concert with her best friend from school. He is hospitalized but Olivia is missing, the mother told ITV television's Good Morning Britain breakfast show.
"I've called the hospitals. I've called all the places, the hotels where people said that children have been taken and I've called the police."
She said she last heard from her daughter just before the concert.
"If anyone sees Olivia, lend her your phone, she knows my number."
There was no immediate claim of responsibility. Supporters of the extremist Islamic State group, which holds territory in Iraq's Mosul and around its de facto capital in the Syrian city of Raqqa, celebrated the blast online. One wrote: "May they taste what the weak people in Mosul and (Raqqa) experience from their being bombed and burned," according to the U.S.-based SITE Intelligence Group.
If the explosion is confirmed as a terrorist attack it would be the deadliest in Britain since four suicide bombers killed 52 London commuters on three subway trains and a bus in July 2005.
Video from inside the arena showed people screaming as they made their way out amid a sea of pink balloons.
U.K. Prime Minister Theresa May and Jeremy Corbyn have agreed that all national campaigning in the General Election were "suspended until further notice" in the aftermath of the incident, according to the Press Association, which cited Labour officials.
May held an emergency COBRA meeting at 9 a.m. Tuesday. The group -- composed of government ministers, military and security chiefs -- typically meets after major incidents to plan the government's overall response. May will also review the U.K.'s threat level, which is currently one below the highest possible level.
May said her thoughts were with victims and their families after what is she said police were treating "as an appalling terrorist attack."
The Dangerous Woman tour is the third concert tour by 23-year-old Grande and supports her third studio album, "Dangerous Woman."
Grande's role as Cat Valentine on Nickelodeon's high school sitcom "Victorious" propelled her to teen idol status, starting in 2010.
After Manchester, Grande was to perform at venues in Europe, including Belgium, Poland, Germany, Switzerland and France, with concerts in Latin America and Asia to follow.
Pop concerts and nightclubs have been a terrorism target before. Most of the 89 dead in the November 2015 attacks in Paris were at the Bataclan concert hall, which gunman struck during a performance by Eagles of Death Metal.
In Turkey, 39 people died when a gunman attacked New Year's revelers at the Reina nightclub in Istanbul.
Manchester, 160 miles (260 kilometers) northwest of London, was hit by a huge Irish Republican Army bomb in 1996 that leveled a swath of the city center. More than 200 people were injured, though no one was killed.
ABC News and The Associated Press contributed to this report.
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