Parsons Green terror attack: Improvised device 'attached to a timer' was used in London tube explosion
Homemade bomb failed to ignite properly and may have been detonated remotely
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A timer was attached to the homemade bomb used in the Parsons Green terror attack, according to reports.
The improvised explosive device may have been detonated on the rush-hour Tube train remotely, security sources told the BBC and Sky News.
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The bomb, which injured at least 22 people, is thought to have failed to properly ignite.
Police are combing through CCTV footage from the London Underground network after launching a manhunt for the person planted the device, which was carried inside a Lidl freezer bag.
Pictures posted on social media appeared to show wires emerging from a bucket filled with chemicals on the floor of the carriage.
Explosive experts told the BBC the images appeared to show "deflagration" - in which a substance burns away rapidly - rather than detonation, which would have killed anyone nearby.
It is believed police could be helped "considerably" by the fact the device was not destroyed by the blast.
Detectives are investigating whether the attacker left the District line train before it reached Parsons Green or fled amid the chaos as the blast sparked a stampede among panicked commuters.
Passengers were "pushed and trampled on" as they sought to escape from the train, according to witnesses.
Most of the 22 people taken to hospital suffered "flash burns" in the explosion, said Metropolitan Police Assistant Commissioner Mark Rowley.
Police have appealed for anyone with photos or videos that may help their investigation to submit them and have set up a web page for material to be uploaded.
Mr Mark Rowley declined to say if any arrests had been made or provide details on the suspects. He said it was a "live investigation".
"There are many urgent inquiries ongoing with hundreds of detectives involved, looking at CCTV, carrying out forensic work and speaking to witnesses," he added.
London Mayor Sadiq Khan told LBC radio station that a "manhunt is underway".
He called for Londoner's to remain "calm and vigilant".
"Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life," Mr Khan added.
Prime Minister Theresa May held an emergency COBRA meeting in Whitehall on Friday afternoon to discuss the bombing, the fifth terror attack to hit the UK this year.