A school bus with no passengers aboard struck and killed a 20-year-old woman in Worcester on Friday evening, police said. The woman, whom police did not identify Saturday, was walking in a crosswalk at the intersection of Southbridge and Cambridge streets when she was hit by the bus, according to a statement from Worcester police. The driver, a 62-year-old woman, told police she did not see the woman in the crosswalk, the statement said. When police arrived, the victim was on the ground with serious injuries, and she was pronounced dead 30 minutes later at a hospital, according to the police statement. The investigation is ongoing and police have not issued any citations.