By Kevin Coombs and Yann Tessier

LONDON (Reuters) – A home-made bomb on a packed rush-hour commuter train in London engulfed a carriage in flames and injured 22 people on Friday in what police said was Britain’s fifth terrorism incident this year, but apparently failed to fully explode.

Passengers on board a train heading into the capital fled in panic as the fire erupted at Parsons Green underground station in West London at 8.20 a.m. (0720 GMT).

Some suffered burns while others were injured in a stampede to escape. The National Health Service said 22 people had been taken to London hospitals, most believed to be suffering flash burns. None were thought to be in a serious condition, the ambulance service said.

“We now assess that this was a detonation of an improvised explosive device,” Britain’s top counter-terrorism officer Mark Rowley told reporters.

Police said officers were making urgent inquiries involving hundreds of detectives backed by the intelligence services to find out who was responsible.

Rowley declined to say if the suspected bomber had been on the train, saying it was a live investigation.

Pictures taken at the scene showed a slightly-charred white bucket with a supermarket freezer bag on the floor of one train carriage. The bucket, still intact, was in flames and there appeared to be wires coming out of the top.

“I was on second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way toward me,” Ola Fayankinnu, who was on the train, told Reuters.

“There were phones, hats, bags all over the place and when I looked back I saw a bag with flames.”

Charlie Craven said he had just got on the train when the device exploded.

“Literally within three seconds of putting your bag down, the doors just closing, we hear a loud explosion,” he told Reuters. “I looked around and saw this massive fireball … coming down the carriage.”

He said terrified passengers fled, fearing a second explosion or a gunman, with people being knocked to the ground and crushed in the stampede to escape.

Outside the station, a woman was carried off on a stretcher with her legs covered in a foil blanket while others were led away swathed in bandages.

“KEEP CALM”

In 2005, 52 people were killed when four British Islamists carried out suicide bomb attacks on three London underground trains and a bus and this year Britain has suffered four attacks which killed a total of 36 people.

Prime Minister Theresa May returned to London to chair a meeting of Britain’s emergency response committee later on Friday.

“My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident,” May said.

Foreign Secretary Boris Johnson said people should “keep calm” and continue their lives as normal.

“Another attack in London by a loser terrorist,” U.S. President Donald Trump said on Twitter. “These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!”

British police have not said anything about who could be behind the attack.

However, a U.S. law enforcement official and a U.S. intelligence source, speaking on condition of anonymity, said the attack might well have been carried out in response to recent Islamic State video messages urging would-be militants to attack trains and other public transport.

The officials said the device “doesn’t look very professionally built” and said its rudimentary design suggested the attack was carried out by someone inspired by Islamic State propaganda rather than by a well-trained cell.

UK security services believe those behind some of the militant incidents in Britain this year had probably been acting alone and likely radicalized by online material.

In March this year, a man drove a car into pedestrians on London’s Westminster Bridge killing four, before he stabbed a policeman to death outside parliament.

A further 22 people were killed in a suicide bombing at a pop concert in Manchester in May and the following month three Islamist militants drove into pedestrians on London Bridge before stabbing people at nearby restaurants and bars, killing eight.

In June, a van was driven into worshippers near a mosque in north London which left one man dead.

On Thursday, figures showed there had been a record number of terrorism-related arrests in the last year and earlier this week Britain’s most senior counter-terrorism officer Mark Rowley said there had been a shift-change in the threat.

In the three years until March this year, police foiled 13 potential attacks but in the next 17 weeks, there were the four attacks while the authorities thwarted six others, Rowley said.

(Additional reporting by Kate Holton, Mark Hosenball, Elizabeth Piper, Paul Sandle, Costas Pitas and Mitch Phillips; writing by Michael Holden; editing by Guy Faulconbridge and Janet Lawrence)