BRENHAM, Texas (AP) _ A second occupant of the car that may have touched off a massive gas explosion last week has succumbed to severe burns, bringing the death toll to three.

Delores Medve, 27, died Sunday at a Houston hospital. She and her mother, Gloria Diver, 46, who died Friday, were in a car that was burned and nearly flattened in the explosion last Tuesday.

Authorities said the vehicle may have ignited the explosion when it drove into a vapor cloud.

The third death was that of a 5-year-old boy killed instantly when the explosion destroyed his family's mobile home. His mother, Jane Meinen, 31, remained in critical condition Monday.

Mrs. Medve's 3-year-old son Travis, who also was in the car, was in stable condition. Sixteen others suffered minor injuries in the explosion, which caused at least $6.6 million in property damage.

The cause of the blast remained unknown Monday.

Lena Guerrero, chairwoman of the Texas Railroad Commission, said investigators believe liquefied petroleum gas products may have been released from a salt dome storage cavern before the explosion.

Mike Benson, spokesman for the National Transportation Safety Board in Washington, said the federal agency also was looking at the cavern and an injection well leading to it.

Investigators previously had been looking at two underground pipelines located in the blast area. But Gary Garrison, railroad commission spokesman, said state officials could not find any problems with the pipelines.