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.
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“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.
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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!”
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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.
