Three people were killed as a passenger van slammed into the back of a school bus early Thursday morning in Corpus Christi in south Texas, local media reported. Three of seven people in the van, including the driver, died on the scene. Four others were sent to hospitals. Their conditions were not immediately known, local TV KRIS reported. There were 12 children on the school bus. Seven were taken to hospitals with unknown injuries. The rest were transported safely to school, according to the report. Investigators said driver fatigue could be a factor in the crash. The van hit the right side of the bus and there are no signs the driver made any attempt to stop before impact. And only one of the survivors was wearing a seat belt.