LONDON — Hundreds of British police embarked on a massive manhunt Sept. 15, 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.
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."
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.
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.
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.
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 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.
(Lori Hinnant in Paris contributed to this report.)