Britain raised its threat level to maximum yesterday and announced troops will be deployed to key sites after a bomb detonated on a packed London Underground train, injuring at least 29 people.<br/><br/>The attack was claimed by the Islamic State terror group. The explosion — Britain's fifth terror attack in six months — sparked a wall of fire that left passengers with burns and caused a stampede of panicking people in which some were trampled. <br/><br/>Twelve hours after the blast at Parsons Green station in southwest London, Prime Minister Theresa May announced the national threat level will be raised to critical, meaning another attack may be imminent. She said, military personnel will take over guard duty at certain closed protected sites, freeing up 1,000 police officers to be deployed on the transport network and on streets across Britain. <br/><br/>The country was last on critical alert after the bombing at a concert in Manchester in May, which was also claimed by the Islamic State terror group. No-one has yet been arrested over the bombing, but anti-terrorism police chief Mark Rowley said the investigation is making good progress.