An oil spill was spreading Monday off the coast of Shenzhen but no explanation was available about how it started or where exactly it came from. The spill started Sunday near a dock at the Yantian Port Area but authorities did not seem to know how much diesel fuel leaked. By Monday, the oil had spread into a 40,000-square-meter slick and was spreading, Wang Shengming, a senior official with the Shenzhen City Maritime Affairs Bureau, told state media. An investigation is underway while clean-up crews use oil-absorbent material to contain the spreading oil. Authorities did not say if the spill posed a threat to people or marine life.