As many as 50 people were treated for chemical exposure, and two people injured in the crash are listed in critical condition. The San Antonio Fire Department, going door to door, found the bodies of an elderly woman and her daughter at a house about a mile from the crash site, but investigators have not determined if they died as a result of exposure to the gas cloud that drifted over the area.
San Antonio Fire Department spokesman Randy Jenkins said the department was "not necessarily making any association" between the deaths of the women and the derailment.
Six people at San Antonio's Sea World attraction 10 miles from the crash site - were treated for chemical exposure.
National Transportation Safety Board Chairman Ellen Engleman Conners said the NTSB will investigate the collision. Jim Remines, investigator-in-charge, is lead the nine-member team, which will include experts in the areas of railroad operations, track, survival factors, mechanical factors, human performance, signals and hazardous materials.