A huge counter-terrorism investigation is under way after a bucket bomb was detonated on a packed London Underground train at Parsons Green. Witnesses described scenes of terror and panic after the blast sent a "fireball" and a "wall of flame" through a District line service in west London. At least 22 people were injured, including a boy believed to be aged about 10. A major manhunt for the bomber has been launched, with police working with MI5 as they forensically examine the device. No arrests have so far been made and the attacker remains at large. After the attack during the Friday morning rush-hour, Britain's most senior counter-terror officer Mark Rowley said: "We now assess this was a detonation of an improvised explosive device."