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.
Police advised the public to avoid the area around the Manchester Arena, and the train station near the arena, Victoria Station, was evacuated and all trains canceled.
Joseph Carozza, a representative of Grande's US record label, said the singer is unharmed and they are investigating what happened.
Ariana Grande released her sentiment on the blast via her Twitter account.
Some of the biggest music artists expressed their sentiments in the said incident.
London mayor Sadiq Khan released a statement via Twitter on the incident.
British Prime Minister Theresa May said the government is working to learn the full details of the blast.
May said the government is trying to establish "the full details of what is being treated by the police as an appalling terrorist attack."
She said her thoughts are with the victims and the families of those who have been affected.
The government is expected to call an emergency cabinet meeting to deal with the incident.
The Dangerous Woman Tour is the third concert tour by Grande to support her third studio album, "Dangerous Woman." The tour began in Phoenix in February.
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. (Franz Correa/SunStar Philippines with AP)