Metropolitan Police Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counterterrorism policing, said the blaze was being treated as a "terrorist incident" and investigators were examining the remains of an improvised explosive device.
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.
The fact that in the three years until March this year, British police foiled 13 potential attacks is an encouraging signal.
The explosion, on a District Line Tube in the British capital's southwest, occurred as the train pulled into Parsons Green station.
Witness Lauren Hubbard described it as "a wall of fire".
"It is very routine in these sort of circumstances for IS to claim responsibility, whether or not they have had any previous engagement with the individuals involved", Rowley said.
At least twenty-two people were sent to the hospital with injuries following the explosion.
"I ended up squashed on the staircase".
"I was on second carriage from the back".
"People were just pushing", he added.
May said the public may see more armed police on the streets and the transport network. "We have remnants of that device".
The site of the blast is in a leafy, affluent part of the city, not near any of London's top tourist sites.More news: Liberum Downgrades Rating On Astrazeneca PLC (AZN)
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British media reported that the bomb included a timer.
"Much of the bucket still seems to be intact and there appear to be no victims with lethal impact wounds", he said.
Islamic State has repeatedly called on its followers to carry out attacks on trains. The attack marks the fifth terrorist incident in the United Kingdom of 2017. That happened after the Westminster Bridge and Borough Market attacks. Similar methods have been used in attacks across Europe, including in Nice, Stockholm, Berlin and Barcelona.
Local residents and businesses rallied together with businesses offering tea and the use of their toilets to people unable to get home. French counter-terrorism officials are investigating the incident.
London Mayor Sadiq Khan said a "manhunt" was underway.
"Since 2010, we've lost hundreds of millions of pounds from the police budget".
The London Underground, which handles 5 million journeys a day, has been targeted several times in the past. The bucket bomb sent a fireball through the subway cart, yet did not fully explode as the creature surely hoped.
U.K. Prime Minister Theresa May criticized President Donald Trump for speculating about the bombing that took place on a London train on Friday morning.
So, as Frankie's mower roared into life, Mr Trump turned and strode to the Oval Office to - he said - place a call to Theresa May. and perhaps try to fix some of that damage.
"I think he means generally that this kind of activity is what we're trying to prevent", McMaster said.