Eighteen people were killed in a suicide bombing at a crowded bus station in Biu, northeast Nigeria, on Thursday, adding to this week's death toll in the restive region.Civilian vigilante Babagana Kyari said "at least 18 people, including three women, died... and several others were injured," an account supported by Ali Dauda who lives locally and saw the blast.The bomber was one of two at the Tashar Gandu motor park on the edge of the town in Borno state. The other was shot dead before he could detonate his explosives, Dauda added.