MANCHESTER, UNITED KINGDOM — At least 19 people were killed and more than 50 people were injured by an explosion that occurred at the end of American pop singer Ariana Grande’s concert in Manchester, UK on Monday night.

The explosion took place as people were leaving Manchester Arena at around 10:30 p.m. The British Transport Police said an explosion took place within the foyer of the concert.

“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,” Majid Khan, 22, one of the witnesses told the Guardian.

The New York Times said a nail bomb appeared to have been used in the attack. Greater Manchester Police said the incident was being treated as terrorist attack until proven otherwise. There was no immediate claim of responsibility.

U.S. officials are drawing comparisons to the coordinated attacks in November 2015 by Islamist militants on the Bataclan concert hall and other sites in Paris, in which about 130 people were killed.

The singer Ariana Grande was reportedly unharmed, according to a representative speaking to NBC News.