1 of 4 | An injured woman is escorted away from 'Parsons Green' station in London on Friday after a terrorist device detonated during the morning rush hour. Photo by Matt Oliver/EPA
Sept. 15 (UPI) -- Police began a manhunt Friday after an explosion rocked a crowded London subway car in what officials say was a terror attack during the morning rush hour.
The blast occurred as a train left Parsons Green station and caused a fire that swept through the subway car. Authorities are investigating the event as a terrorist-related attack, London Metropolitan Police said.
Government officials raised the terror threat level in England from "severe" to "critical," which means an attack is "expected imminently." The last time the threat level was raised to critical was the day after the Manchester terror bombing in May.
Several passengers were burned and dozens hurried to leave the train and get out of harm's way. At least 29 people were injured, with NHS England reporting 21 still in the hospital by late Friday.
Passengers on the train said a wall of fire erupted aboard one car.
A major manhunt for the bomber was launched and police asked for information from any witnesses or other sources.
Police have not yet made any arrests. London Mayor Sadiq Khan confirmed the manhunt but did not say if any specific person is being sought.
Although reports have claimed police are pursuing a suspect based on images from CCTV at the scene, Scotland Yard told The Standard that "there's no truth to that."
Metropolitan Police Assistant Commissioner Marc Rowley said an improvised explosive device caused the explosion. Authorities added that the IED may not have fully detonated, and might have been set off with a timer.
Rowley also said British counterintelligence agency MI5 is involved in the investigation.
The incident prompted panic in the subway, disrupted schedules and drew a quick and heavy response from rescue workers and armed police. Emergency responders were on the scene within minutes, Rowley said.
The attack drew swift condemnation from U.S. officials.
"Another attack in London by a loser terrorist," President Donald Trump tweeted early Friday. "These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"
"The U.S. remains committed to defeating terrorist organizations as well as their evil ideology," Trump's national security adviser, H.R. McMaster, said at a White House press briefing Friday afternoon.
British officials criticized Trump's tweets as "unhelpful" and "dangerous," although the president expressed sympathies and offered help in a phone call with British Prime Minister Theresa May Friday.
Friday's was the fifth terrorism-related attack in London this year. The previous four killed 36 people, and police said six other significant plots, soon to be prosecuted, were thwarted before they occurred.