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A n investigation and manhunt got underway Friday morning following a subway explosion in London that was being treated as a terror attack.

The blast occurred at Parsons Green Tube Station shortly after 8 a.m. local time, According to Mark Rowley, Assistant Commissioner for London’s Metropolitan Police Service. At least 22 people have been hospitalized, though the London Ambulance service said no injuries were considered serious or life-threatening, according to ABC News.

Eyewitness Martin Adams said he was riding in the subway at the time of the attack.

I heard a loud bang and as I looked to my right, there was a flame, a fireball came through the carriage,” he told ABC News. “As the doors opened, people then began leaving the train straight away. I saw some flames coming from what I thought was a blue bag.”

Sally Faulding said she saw people with burns as she watched panic ensue on the subway platform.

I also saw people injured obviously from having been stampeded on the platform because we were all running,” she said. “People were falling over.”

The cause of the blast was an “improvised device” that failed to fully explode, police said. So far, no arrests have been made.

Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” London Mayor Sadiq Khan said in a statement. “As London has proven again and again, we will never be intimidated or defeated by terrorism.”

President Donald Trump responded to the news via Twitter at 6:42 a.m. EST, calling those responsible for the attack “sick and demented people.”

This story is developing.

SOURCE: ABC News, Associated Press

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