A homemade bomb on a packed rush-hour commuter train in London engulfed a carriage in flames and injured 29 people - but it apparently failed to fully explode in Britain's fifth major terrorism incident this year.
Passengers heading into the British capital fled in panic after the blast, as the train was about to depart Parsons Green station in West London at 8:20am (local time).
Some suffered burns and others were injured in a stampede to escape the station, one of the above-ground stops on the underground network, but health officials said none were thought to be in a serious condition.
"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 a hunt involving hundreds of detectives backed by the intelligence services was under way 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.
Prime Minister Theresa May called the incident a "cowardly attack" and said the national threat level had been raised to its highest level, critical".
Islamic State claimed responsibility through its news agency, Amaq.
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 towards me," Ola Fayankinnu, who was on the train, said.
"There were phones, hats, bags all over the place and when I looked back I saw a bag with flames."
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 that killed a total of 36 people.
"Another attack in London by a loser terrorist," US President Donald Trump said on Twitter. "These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"
May, who returned to London to chair a meeting of Britain's emergency response committee, said police and security services were working to track down those responsible.
Asked about Trump's tweet, she replied: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."
A US law enforcement official and a US 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.
One of the officials said the device "doesn't look very professionally built" and said its rudimentary design suggested the attack had been carried out by someone inspired by Islamic State propaganda rather than by a well-trained cell.