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LONDON (CBSMIAMI) — Police in London have launched a manhunt after an explosion on a crowded subway Friday morning. Emergency officials say an improvised explosive device detonated on the train and injured at least 22 people.

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The Metropolitan Police force says police “are making fast-time inquiries to establish who was responsible.” Counterterrorism policing chief Mark Rowley says hundreds of detectives are looking at surveillance camera footage, carrying out forensic work and speaking to witnesses.

Police were called to the Parsons Green station in southwest London after a woman on the platform saw what she says was an explosion.

“Suddenly, I heard a big bang and I turned left and I saw the fireball,” explained witness Shu-chen Warner.

Cell phone video from the subway car shows a white plastic bucket inside a shopping bag, with what appear to be wires and a flame.  Sources tell CBS News the improvised explosive device had a timer and didn’t fully detonate.

“We managed to pull ourselves away and literally just jump over the fence and the run down the track as fast as we can,” said passenger Charlie Craven.

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Armed police shut down part of the train line and locked down schools in the southwest London neighborhood.  Witnesses say children were on board when the blast went off.

“There was an unfortunate little boy who smacked his head on the ground – they went flying,” described one witness.

“I heard a large bang from the doors on the other side of the train and um this fire ball above my head singed all my hair, there were burn marks on the top of my head….and everyone just ran off the train. It was scary,” said another witness.

Police tweeted a request to Londoners after the incident, writing in part: We ask everyone to remain vigilant and to report any suspicious behavior.

British Prime Minister Theresa May will lead a national security meeting Friday in London.

May also tweeted: My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.

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President Trump also tweeted after the explosion, “Another London attack by a loser terrorist.” In another tweet, Mister Trump suggested the terrorists were known to British police. A Scotland Yard spokesman called the comment “unhelpful speculation.”