LONDON — Alex Ojeda-Sierra, 13, was on the train to school with a friend when they heard screaming and saw passengers running past.
Unknown to the boys, a bomb had exploded in another car.
“I dropped my bag and we started running,” Alex, who attends the London Oratory School, said from a wheelchair at Chelsea and Westminster Hospital, where he was treated for facial bruises and sprains when he tripped in the panicky crush of fleeing commuters.
“One man fell on me and I had my legs bent backwards and my right ankle got twisted and I started screaming that I had no air,” he said.
The bomb, wrapped in a plastic grocery bag concealed in a bucket, exploded at 8:20 a.m. Friday at the height of the morning rush. The explosion and panic left 29 people injured, but none were killed.
It was the fifth terror attack in Britain this year and the first to hit London at its most vulnerable point — mass transit — since the 2005 bombings that killed 52.
The Islamic State asserted responsibility for the bucket bomb hours later in a message on its Amaq news site that said a “detachment” of its disciples had carried out the attack — language that suggested more than assailant.
Prime Minister Theresa May, calling the blast a “cowardly attack,” said the national threat level had been raised a notch to “critical,” the highest.
The bomb exploded just after the train drew into Parsons Green, an elevated station in a quiet and affluent part of West London. It burned at least one passenger, who was carried away on a stretcher, and led to a stampede that injured others.
The attack revived the specter of mass casualties from terrorism on the London Underground, commonly known as the Tube, the world’s oldest subway system and one of the busiest. Though one would-be attacker tried to bomb an Underground train in 2016, the device failed to detonate.
The most recent attackers in London, and across Europe, have instead used vans and cars as weapons to crush and maim people.
The head of security on the Underground at the time of the 2005 attacks, Geoff Dunlop, said it was unsurprising that terrorism had returned to the Underground.
“You can do an awful lot to make it safer but you can never totally secure it because of the very nature of it,” said Mr. Dunlop, who left the Underground in 2013 and now works as a private security consultant. “It has to be open.”
Witnesses on the train described a tremor, a wave of heat and then a barrage of flames that quickly dissipated.
“The train was packed, and I was down the other side of the carriage standing up, looking at my phone and then I heard a big boom and felt this heat on my face,” said Natalie Belford, 42, a hairdresser and beautician who was on the train. “I ran for my life, but there was no way out. The doors were full of people and the carriage was too packed to move down.”
London’s mayor, Sadiq Khan, a face of resolve following the earlier attacks, issued a defiant statement on Facebook that hinted at how terror attacks had become a new normal in the capital.
“Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” Mr. Khan wrote. “As London has proven again and again, we will never be intimidated or defeated by terrorism.”
The police were combing through the extensive CCTV footage that blankets all Underground stations, with particular attention to the handful of stations to the west of Parsons Green.
It was unclear by Friday night whether any possible suspects had been identified from CCTV surveillance.
Roy Ramm, a former commander of specialist operations at Scotland Yard, said that police would be undertaking a comprehensive forensic examination of the train and the device to determine what happened.
“The police will also ask witnesses the age-old question: Did anyone see what happened?” he said. “They will be investigating what the detonation mechanism was and going through CCTV footage to see who might be behind it.”
The device did not appear to detonate properly, as the bucket, the bag and a series of wires all remained intact even after the explosion.
Many of the victims seemed to have been hurt by the panicked rush to escape the scene, rather than the explosion itself. Ms. Belford, the hairdresser, said she was knocked over twice, and showed a reporter her ripped tights and bloodied knee.
While Londoners are largely hardened to terrorism and determined to defy the threat, Uber use in west London surged to three times its normal rate after the attack, the company said. But it was unclear whether passengers were turning to the ride service out of fear or practicality; part of the Underground system was suspended following the attack.
Lorna Fletcher, a resident of the Parsons Green neighborhood, said she already tried to avoid using the subway precisely because of the possibility of this kind of event.
“At the weekends I always try and get my friends to come out here – that way I don’t have to travel far and it’s not playing on the back of mind,“ Ms. Fletcher said. “It was only a matter of a time before the trains would get attacked.”
The suburban setting of the attack, several miles west of downtown London, sparked debate about whether the bomb had been mistakenly detonated prematurely – or was purposely meant to highlight how no part of the city is safe.
“Parsons Green is not emblematic or symbolic, and I think that will be a puzzlement for investigating officers, who will ask: Was it intended to be detonated or did it go off there by accident?” Mr. Ramm said. “If you look at a list of target areas in London, Parsons Green would not be in the top 100.”
Local residents echoed Mr. Ramm’s surprise. “My American friends just moved out here with their children because we told them it was one of the safest parts of London. I feel foolish now,” said Rachel Palmer, 34, a mother of two. “I just never imagined it would happen out here.”
Tensions were heightened in the neighborhood after police sealed off several nearby streets and buildings, including Lady Margaret’s school for girls, where students were kept under lockdown for hours.
Life nevertheless continued largely as normal across most other parts of London. While the 2005 attacks led to a shutdown of large parts of the subway network, only a small section close to Parsons Green was suspended on Friday. Large crowds of tourists gathered outside Buckingham Palace, the official residence of the Queen, Elizabeth II.
Hugh Coyne, a physician at a medical practice near the station, said he went to the scene to offer help to emergency workers, only to be told there were no major injuries to treat.
The attack led to tensions between President Trump and the British authorities, who were annoyed that Mr. Trump had appeared to imply in a tweet that the attacker or attackers were known to Scotland Yard, the headquarters of the Metropolitan Police, London’s law enforcement agency.
Mrs. May said Mr. Trump’s statement was unhelpful and the Metropolitan Police said it was “pure speculation.”
After a terror attack in Manchester this summer, American officials angered their British counterparts by leaking information shared with them by the British police.
In June, Mr. Trump was also entangled in a separate spat with the London mayor, Mr. Khan, in which he criticized Mr. Khan’s response to an attack on London Bridge that left seven dead.