TIANJIN, China — This bustling port city a 90-minute drive from the Chinese capital awoke on Thursday to scenes of devastation — huge gray plumes of smoke, vast parking lots of burned-out vehicles, block after block of high-rises with their windows blown out — and unanswered questions about what caused a pair of enormous explosions whose terrifying fury was broadcast around the world.

As the death toll climbed to 50 from the blasts on Wednesday night, rescue workers combed the rubble of the city's flattened warehouse district for bodies while hundreds of people crowded hospitals. Throughout the day, hundreds more lined up to donate blood in the wilting heat.