At least 22 people were killed and 59 wounded in an explosion at the end of a concert by U.S. singer Ariana Grande in the English city of Manchester on Monday, in what two U.S. officials said was a suspected suicide bombing, REUTERS reports.

Prime Minister Theresa May said the incident was being treated as a terrorist attack. If confirmed, it would be the deadliest militant assault in Britain since four British Muslims killed 52 people in suicide bombings on London’s transport system in July 2005.

Police responded to reports of an explosion shortly after 10:33 pm (2133 GMT) at Manchester Arena, which has the capacity to hold 21,000 people, where the U.S. singer had been performing to an audience that included many children.

A witness who attended the concert said she felt a huge blast as she was leaving the arena, followed by screaming and a rush by thousands of people trying to escape the building.

A video posted on Twitter showed fans, many of them young, screaming and running from the venue. Dozens of parents frantically searched for their children, posting photos and pleading for information on social media.

“We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming,” concert-goer Catherine Macfarlane told Reuters.

“It was a huge explosion – you could feel it in your chest. It was chaotic. Everybody was running and screaming and just trying to get out.”

Ariana Grande, 23, later said on Twitter: “broken. from the bottom of my heart, i am so so sorry. i don’t have words.” May, who faces an election in two-and-a-half weeks, said her thoughts were with the victims and their families. May and Jeremy Corbyn, the leader of the opposition Labour Party, agreed to suspend campaigning ahead of the June 8 election.

“We are working to establish the full details of what is being treated by the police as an appalling terrorist attack,” May said in a statement. “All our thoughts are with the victims and the families of those who have been affected.”

May will hold a crisis response meeting on Tuesday.

Manchester Chief Constable Ian Hopkins said police were treating the blast as a terrorist incident and were working with counter-terrorism police and intelligence agencies but gave no further details on their investigation.