By JILL LAWLESS and GREGORY KATZ
LONDON (AP) - Hundreds of British police embarked on a massive manhunt Friday, racing to find out who placed a homemade bomb on a packed London subway train during the morning rush hour.
The explosion - labeled a terrorist attack by police - wounded 22 people and ignited a panicked stampede to safety. Experts said London may have escaped far worse carnage because it appeared that the bomb only partially exploded.
"Clearly, this was a device that was intended to cause significant harm," Prime Minister Theresa May said after chairing a meeting of the government's COBRA emergency committee.
Police called it a terrorist attack, the fifth in Britain this year.
Witnesses described seeing a "wall of fire" as the bomb - hidden in a plastic bucket inside a supermarket freezer bag - went off about 8:20 a.m. while the train was at the Parsons Green station in southwest London.
It was not a large explosion, and British police and health officials said none of the injured was thought to be seriously hurt.
The Metropolitan Police force said there had been no arrests so far, but hundreds of detectives, aided by intelligence agents, were looking at surveillance camera footage of the subway, carrying out forensic work and speaking to witnesses.
It's not clear whether the device was intended to explode when it did. The site of the blast was in a leafy, affluent part of the city, not near any of London's top tourist sites. British media reported that the bomb included a timer.
Photos taken inside the train show a white plastic bucket inside a foil-lined shopping bag. Flames and what appear to be wires emerge from the top.
Terrorism analyst Magnus Ranstorp of the Swedish Defense University said that, from photos, it appeared the bomb did not fully detonate, as much of the device and its casing remained intact.
"They were really lucky with this one, it could have really become much worse," he said.
Police were first alerted when commuters reported a noise and a flash aboard the District Line train. Commuter Lauren Hubbard was on the train when she heard a loud bang.
"I looked around and this wall of fire was just coming towards us. You just run," said Hubbard, who fled the above-ground station with her boyfriend.
Others described "absolute chaos" as hundreds rushed to flee the danger.
"I ended up squashed on the staircase. People were falling over, people fainting, crying, there were little kids clinging onto the back of me," said Ryan Barnett, 25.
Mark Rowley, head of counterterrorism for the Metropolitan Police, said "this was a detonation of an improvised explosive device."
He said 18 people had been injured, most with "flash burns." Health officials later said four others hurt in the bombing went to the hospital themselves.
Rowley said Britain's domestic intelligence service, MI5, was helping with the investigation, which was being led by the police counter-terrorism unit. He gave no information about potential suspects, saying "It's very much a live investigation."
U.S. President Donald Trump tweeted that it was another attack "by a loser terrorist," adding that "these are sick and demented people who were in the sights of Scotland Yard."
The London police force declined to comment on Trump's suggestion that it knew about the attacker. May said it was not helpful "for anybody to speculate" on an ongoing investigation.
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."
He said many of those on board were schoolchildren, who were knocked around as the crowd surged away from the fireball.
Commuter Richard Aylmer-Hall said he saw several people injured, apparently trampled as they fled.
"I saw crying women. There was lots of shouting and screaming, there was a bit of a crush on the stairs going down to the streets," he said.
During rush hour, the London Underground train can hold more than 800 people. Trains were suspended along a stretch of the line, and several homes were evacuated as police set up a 50-meter (yard) cordon around the scene while they secured the device.
London Mayor Sadiq Khan said the city "utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life."
May said that Britain's official threat level from terrorism remained at "severe," meaning an attack is highly likely, and was not being raised to critical.
The country's threat level was briefly raised to critical, meaning an attack may be imminent, after a May 22 suicide bombing at Manchester Arena that killed 22 people.
Friday's attack is the fourth in London this year, after deadly vehicle attacks near Parliament, on London Bridge and near a mosque in Finsbury Park in north London.
British authorities say they have foiled 19 plots since the middle of 2013, six of them since a van and knife attack on Westminster Bridge and Parliament in March, which killed five people.
The London Underground has been targeted several times in the past, notably in July 2005, when suicide bombers blew themselves up on three subway trains and a bus, killing 52 people and themselves. Four more bombers tried a similar attack two weeks later, but their devices failed to fully explode.
Last year Damon Smith, a student with an interest in weapons and Islamic extremism, left a knapsack filled with explosives and ball bearings on a London subway train. It failed to explode.
In its recent Inspire magazine, al-Qaida urged supporters to target trains.
Separately, French counterterrorism authorities were investigating an attempted knife attack Friday on a soldier patrolling a large Paris subway interchange.
The knife-wielding assailant tried to attack a soldier assigned to a special military force protecting prominent sites following deadly Islamic extremist attacks. He was quickly arrested and no one was hurt.
Lori Hinnant in Paris contributed to this report.
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