MANCHESTER, England -- At least 19 people are dead and 50 injured in a possible terrorist incident Monday night at Manchester Arena in England, where pop singer Ariana Grande was performing, Greater Manchester Police said.
Greater Manchester Police Chief Constable Ian Hopkins said police received reports of an explosion at the conclusion of the Ariana Grande concert, and confirmed 19 people were killed and 50 others treated at area hospitals.
"We are currently treating this as a terrorist incident until we have further information," Hopkins said during a brief statement.
Hopkins said the police department is working with local and national agencies in their investigation.
The incident happened shortly after Grande had left the stage, shortly after 10:30 p.m. (4:30 p.m. CT) according to eyewitnesses. The arena previously said on Twitter that the incident took place outside the venue in a public space.
Grande expressed remorse on Twitter after the incident, tweeting "i am so so sorry. i don't have words."
Calvin Welsford, an 18-year-old concertgoer, said that he heard a "loud bang" a couple of minutes after Grande's set had finished.
"Ariana was off the stage, she'd finished her set," he told CNN. "I looked around because I heard this big bang, people screaming and crying and flooded towards the stage."
Investigators are looking into the possibility of a suicide bombing outside of the arena, according to a Western law enforcement official and a US law enforcement official. A male at the scene in Manchester has been identified as the probable bomber, and a US official said suicide bombing is now considered to be the "likely" reason for the blast.
After the initial incident, police carried out a precautionary controlled explosion nearby but the item turned out to be discarded clothing, not a "suspicious item," Greater Manchester Police said.
Earlier police had warned people to stay away from Manchester Arena after reports of two loud explosions. The crowd was made up of mainly younger people who had come to see Grande in her first of three scheduled concerts in the UK, on her European and Latin American tour.
Ivo Delgado, a witness, told CNN he heard one explosion after the concert ended. He said there was smoke in a main corridor outside the arena seating and stage area as he and others were heading out of the building.
"It was a really big explosion. Everybody started screaming and running," he said.
Delgado said: "There were people on the floor ... There was a lot of confusion."
Simran Rayarel, 16, was in the VIP area when she heard the blast. "I heard the bang (and) everyone froze. Then madness broke out," she said.
"I saw everyone running, I fell onto the seats on the floor my friend lifted me up."
Social media posts from the scene showed panicked concertgoers running down the arena's stairs in an attempt to get away.
British Transport Police told CNN it was in the process of closing some transport in the area.
Video from near the scene showed a caravan of ambulances heading to the arena.
Universal Music Group, parent company for Grande's record label, Republic Records, posted a statement on Facebook.
"We are deeply saddened to learn of tonight's devastating event in Manchester. Our thoughts and prayers are with all those affected by this tragedy," the post says.
British Prime Minister Theresa May issued a statement, confirming that police are treating the incident as a terror attack.
"We are working to establish the full details of what is being treated by the police as an appalling terrorist attack. All our thoughts are with the victims and the families of those who have been affected."
The leader of Britain's Labour Party, Jeremy Corbyn, offered support on Twitter. "Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services."
Musicians such as Ed Sheeran, Nicki Minaj and Katy Perry all offered supportive tweets.
Perry tweeted Monday that she is "Praying for everyone at @ArianaGrande's show."