UK threat level 'critical'

A homemade bomb planted in a rush-hour subway car exploded in London on Friday, injuring 29 people and prompting authorities to raise Britain's terrorism threat level to "critical," meaning another attack may be imminent.

The early morning blast sparked a huge manhunt for the perpetrators of what police said was the fourth terrorist attack in the British capital this year.

Prime Minister Theresa May, acting on the recommendation of the Joint Terrorism Analysis Center, raised the country's threat level from "severe" to "critical" — its highest possible level. May said military troops would augment the police presence in a "proportionate and sensible step."

Earlier, May said the device had been "intended to cause significant harm."

Still, to the relief of authorities and Londoners, experts said the bomb — hidden in a plastic bucket inside a supermarket freezer bag — only partially exploded, sparing the city much worse carnage.

"I would say this was a failed high-explosive device," Chris Hunter, a former British army bomb expert, said of the blast, which caused no serious injuries.

The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit.

The bomb went off around 8:20 a.m. as the train, carrying commuters from the suburbs — including many school children — was at Parsons Green station in the southwest of the city.

Witness Chris Wildish told Sky News that he saw "out of the corner of my eye, a massive flash of flames that went up the side of the train," followed by "an acrid chemical smell."

Commuter Lauren Hubbard said she was on the train when she heard a loud bang.

"I looked around and this wall of fire was just coming toward us," Hubbard said. She said her instinct was "just run," and she fled the above-ground station with her boyfriend.

Chaos ensued as hundreds of people, some of them suffering burns, poured from the train, which can hold up to 800 people.

The Metropolitan Police said hundreds of detectives, along with agents of the domestic spy agency MI5, were looking at surveillance camera footage, carrying out forensic work and speaking to witnesses.

Speaking to reporters late Friday, Assistant Commissioner Mark Rowley said police were making "good progress" and that the public should be reassured that more police and troops will be on the streets.