The security video showed a London subway car crowded with midday riders, a man with a backpack among them. Suddenly, a large puff of dust shot out of his pack. British prosecutors played the video to a jury in London on Tuesday. The man was Ramzi Mohammed, 25, one of six men on trial for conspiracy to murder commuters on July 21, 2005. The dust was from a bomb hidden in his backpack. Its detonator fired but the shrapnel-laced main charge failed to explode.