Updated at 4 p.m. ET
British Prime Minister Theresa May called Friday's morning rush-hour attack on a London subway train that wounded at least 29 people a "cowardly" act and raised the country's threat level to critical.
London's Metropolitan Police are investigating the explosion on the train at the Parsons Green station in the capital, calling it an act of terrorism. The Islamic State's Amaq news agency said the militant group was responsible for the attack, but that claim could not be independently verified.
"We can confirm that we treated a total of 19 patients — mostly for minor injuries — and took them to three London hospitals" after the incident at the Parsons Green metro station," said Natasha Wills of the London Ambulance Service.
Ten other patients went to London hospitals on their own following the incident at the aboveground station, the National Health Service says. Of the 29 people who went to hospitals, 21 were listed as "currently being treated" Friday evening (local time).
No deaths have been reported as a result of the blast, which seems to have left a small fire burning in a bucket inside the train car. Emergency calls went out at 8:20 a.m. local time, when a fire was reported at the Parsons Green station on the London Underground.
"There was an explosion on a tube train," Assistant Commissioner Mark Rowley of the Metropolitan Police Service said at a news briefing. "We now assess that this was a detonation of an improvised explosive device."
Of the injuries sustained, Rowley said, "I understand most of those to be flash burns."
Photos of some of the people who were in the area of the blast show singed hair and skin reddened by intense heat.
When Rowley was asked whether a suspect is in custody — or whether it's known if the person responsible for the device was still on the train when it exploded — he stated that the investigation was ongoing and that hundreds of detectives are working on it.
NPR's Frank Langfitt reports for our Newscast Unit:
"Photos posted on social media show a white plastic bucket with flames coming out of the top — along with wires. The bucket is sitting next to a subway train door. A purse, apparently abandoned by a fleeing passenger, sits in the foreground.
"The explosion occurred at the Parsons Green station in Fullham, which is about 4 miles southwest of Big Ben."
Providing more details about the blast, Metro.co.uk reports:
"A metro.co.uk reporter at the scene described seeing people with facial burns, adding that they were 'really badly burned' and 'their hair was coming off.'
"Fire crews and paramedics also rushed to the scene to help people after the fireball went down the carriage.
"They said that the rear of the train filled with smoke and people left the train, some panicking, at Parsons Green."
Witnesses say hundreds of people rushed to get out of the station. Police are asking members of the public to share any images and information they might have.
"The Metropolitan Police have confirmed that the explosion on a train at Parsons Green Station this morning is being treated as terrorism," London Mayor Sadiq Khan said. "Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life."
President Trump said via a tweet, "Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"
The United Kingdom has suffered four terrorist attacks since March, including vehicle attacks on London and Westminster Bridges and a bombing outside an Arianna Grande concert in Manchester.
This is a breaking news story. As often happens in situations like these, some information reported early may turn out to be inaccurate. We'll move quickly to correct the record, and we'll only point to the best information we have at the time.