U.K. police arrested a man Saturday in connection with the London Underground train attack.
Police confirmed an explosion went off on a train at the Parsons Green District Line station around 8:20 a.m. local time Friday. Officials said an improvised explosive device caused the explosion. They didn't release additional details.
ISIS claims a "detachment" from the terror group carried out the attack. Authorities haven't released information about the 18-year-old suspect, but called the arrest "significant."
As of early Saturday morning, most of those people had been released.
London's terror threat level has been raised to critical. Mayor Sadiq Khan urged citizens to "remain calm," saying London "will never be intimidated or defeated by terrorism."