LONDON (Updated) -- Greater Manchester Police said 22 people died in an attack on concert-goers at an Ariana Grande performance in northern England late Monday, May 22.
Chief Constable Ian Hopkins said Tuesday the attacker used an explosive device. Police are trying to determine if the attacker acted alone or had support.
Police said some 400 officers were deployed overnight to help with the investigation.
Officials said children were among the victims of Monday's explosion.
There was mass panic after the explosion at the end of the concert, which was part of Grande's "The Dangerous Woman" Tour. The singer was not injured, according to a representative.
Britain's terrorist threat level has been set at "severe" in recent years, indicating an attack is highly likely. Police said the explosion is being judged as a terrorist attack unless new information proves otherwise.
Witnesses reported hearing two loud bangs coming from near the arena's bars at about 10:35 p.m. but there were few further details.
"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena," concertgoer Majid Khan, 22, told Britain's Press Association. "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."
Oliver Jones, 17, said, "The bang echoed around the foyer of the arena and people started to run."
Video from inside the arena showed concertgoers screaming as they made their way out amid a sea of pink balloons.
EXPLOSION AT MANCHESTER ARENA AND EVERYONE RAN OUT SO SCARY?? pic.twitter.com/pJbUBoELtE— ?? (@hannawwh) May 22, 2017
broken.— Ariana Grande (@ArianaGrande) May 23, 2017
from the bottom of my heart, i am so so sorry. i don't have words.
Praying for everyone at @ArianaGrande's show— KATY PERRY (@katyperry) May 22, 2017
My heart hurts for my sister, Ariana & every family affected by this tragic event in the U.K. Innocent lives lost. I'm so sorry to hear this— NICKI MINAJ (@NICKIMINAJ) May 23, 2017
My thoughts, prayers and tears for all those affected by the Manchester tragedy tonight. I'm sending all my love.— Taylor Swift (@taylorswift13) May 23, 2017
MY PRAYERS GO OUT TO PPL OF MANCHESTER…HAD SPECIAL TIMES THERE FROM YOUTH & BEYOND— Cher (@cher) May 22, 2017
Just saw the news about Manchester... Our hearts are breaking ?? Prayers for all who attended, their families, Ari and her whole crew.— Fifth Harmony (@FifthHarmony) May 23, 2017
Just hearing about the explosions at the Arianna concert.. Praying for everybody!! Sick..— Blake Shelton (@blakeshelton) May 22, 2017
London stands with Manchester - our thoughts are with all those killed and injured tonight and our brave emergency services. https://t.co/0al1LzXvXE— Sadiq Khan (@SadiqKhan) May 22, 2017