UK terror threat level raised to 'critical' after London Tube explosion

The terror threat in the United Kingdom has been raised from severe to critical, Prime Minister Theresa May announced, a spokesperson told CNN. The terror threat level of "critical" means than another attack is considered to be "imminent."
A major manhunt is under way after an improvised explosive device exploded on a London Underground train, injuring 29 people in what police have called a terrorist incident.
None of the injured are thought to be in a serious or life-threatening condition, according to the London Ambulance Service, which took 19 of the injured to hospitals. Ten others went to hospitals on their own. The blast occurred during the morning rush hour at about 8:20 a.m. local time (3:20 a.m. ET) Friday at Parsons Green Tube station, a leafy stop on a busy commuter line from southwest London into the center of the capital.
We now assess that this was a detonation of an improvised explosive device," London's Metropolitan Police Assistant Commissioner Mark Rowley said at a news conference. Most of the injuries appear to be the result of flash burns, he said.
Asked by a reporter if police had any information on the perpetrator, Rowley said: "It's a live investigation."
No arrests have been made so far. Hundreds of detectives are working on the inquiry with the support of Britain's domestic intelligence service, MI5.