Responding to a presumed terrorist attack in Manchester, England, in which at least 22 people, including children, were killed Tuesday night when a suicide bomber detonated explosives during a concert by pop star Ariana Grande, the local bishop has said that such a thing "can have no justification".
The Arena seats about 21,000 people.
The investigation is "complex and wide ranging", police said, as they urged people not to speculate on who the attacker was.
A spokesman for the singer had earlier said she had been uninjured in the incident.
Eye witnesses spoke of hearing an explosion which sparked panic, as adults and children, many in tears or screaming, ran for their lives from the danger.
One described it as a stampede, with people being trampled on. Workers with China's Consulate General in Manchester, who rushed to the scene after the explosion, told Xinhua over phone that the area had been cordoned off with a heavy presence of police vehicles in sight.
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Police were called to reports of an explosion at the Manchester Arena at 10.33pm, shortly after USA singer Ariana Grande had finished her performance.
Robinson took dozens of teenage girls to the nearby Holiday Inn Express hotel and tweeted out her phone number to anxious parents, telling them to meet her there. Emergency vehicles were helping the injured and bomb disposal units were later seen outside the venue. A controlled explosion was carried out on a second suspect device close by in a garden close to the city's cathedral.
In addition some city residents opened their homes to provide overnight lodging for people who were stranded by the shutdown in some train services because of the incident.
Regional ambulance services raced to the scene to take casualties, many of them seriously injured, to hospitals.
Prime Minister Theresa May said the incident was being treated as a terrorist attack. The government is expected to call an emergency Cabinet meeting.
"However, the public may experience increased security in and around public places and events as officials take additional precautions", it said.
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Mr Walker told BBC Radio 5 Live he was around three metres from the explosion and was "surprised I got away so lightly".
"A concert hall where young people are enjoying music - and nobody would contemplate anything like this happening", he added. With over one million visitors per year, the venue claims to be "one of the busiest venues in the world and the largest indoor arena in Europe", according to its website.
The 23-year-old U.S. pop singer, who found fame as the child star of Nickelodeon series Victorious, was in the middle of a world tour.
After Manchester she was due to play two nights in London's O2 Arena before visiting Poland, Germany, Switzerland, France, Spain and Italy.
Others offered rides and rooms to those in the area using the hashtag #RoomforManchester.
Manchester terror attack: What we know
She called the singer "a dope soul inside and out". "It was sheer panic", he said. "I just want her to walk through the door". There was mass panic after the explosion at the end of the concert, which was part of Grande's The Dangerous Woman Tour.