A rush-hour explosion on a Shanghai bus killed at least three people Monday, state media said. The official Xinhua News Agency said the three died in the blast at about 9 a.m. local time according to firefighters at the scene. Xinhua said the cause of the explosion was not yet known. It said the firefighters estimated that about 50 passengers were on the No. 842 bus in the Yangpu district in northeast Shanghai at the time of the blast.