A huge manhunt is underway for those responsible for a terror attack that sparked panic on a packed London Tube train and left 29 people injured though, miraculously, no one was killed.

Isis has claimed responsibility for the attack, which happened during the rush hour commute when an improvised explosive device was detonated sending a “fireball” through the train carriage at Parsons Green station in south-west London.

Police reportedly know the identity of the suspected bomber and officers are “chasing down suspects”.

Late on Friday night, Theresa May raise the terror threat level to critical, the highest level, which means authorities think another attack may be “imminent”.

It is the fourth time the assessment has been placed at the highest level for the past 11 years.

Passengers, including children, fled the District Line train after the attack, which took place at around 8.20am.

Eyewitnesses described a “fireball” and a “wall of flame” through the packed London Underground train after the blast, as one commuter described how his hair was left singed by the blast.

A huge investigation led by the Metropolitan Police’s Counter Terrorism Command is now underway, with hundreds of detectives carrying out enquiries including examining CCTV, forensic work and speaking to eyewitnesses.

The Amaq news agency, the media arm of Isis, has claimed responsibility for the attack, though there is no concrete evidence the jihadist group directed the attack.

According to Sky News’ Home Affairs Correspondent Mark White, security sources say they have identified a suspect involved in the bombing with the help of CCTV footage.

Met Police Assistant Commissioner Mark Rowley suggested there may have been more than one person involved stating that police were “chasing down suspects”.

In a statement outside Scotland Yard, the UK’s most senior anti-terror police officer said: “Somebody has planted an improvised explosive device on the Tube – we have to be open-minded at this stage about him and about potential associates.”

Injuries – 29 people are being treated in hospital after the blast which sent a “fireball” and a “wall of flame” through a packed London Underground train (Picture: Dominic Lipinski/PA Wire) More

Bucket bomb – eyewitnesses took pictures of a bucket flickering with flames (Picture: Sylvain Pennec/PA Wire) More

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