At least 19 are dead and around 50 injured after a blast at a Ariana Grande concert in Manchester, UK on May 22.
British police said it is being treated as a terrorist incident.
Police said they responded to reports of an explosion.
Manchester Arena has a capacity for 21,000 people. It is the largest indoor arena in Europe.
One witness said she felt a huge blast as she was leaving the arena. Then came screaming and a rush as thousands of people trying to escape.
Other witnesses said there were many children at the concert.
Britain’s threat level is at “severe,” which means a terrorist attack is considered highly likely.