By Terri Jo Neff |

Three investigators with the National Transportation Safety Board are in Coolidge hoping to determine what caused a 30-inch El Paso Natural Gas pipeline to explode earlier this week, killing two members of a family and critically injuring a third.

Rosalita Alvarez remains hospitalized in Phoenix after suffering severe burns in a house fire which ignited shortly after 5:30 a.m. last Sunday. Her husband Luiz and their daughter Valeria, 14, died inside the house, according to the Pinal County Sheriff’s Office.

Authorities believe the fire was triggered by the explosion of the Kinder Morgan-owned pipeline. The ruptured portion of the pipeline is located about one football field’s distance from the Alvarez home, which was destroyed in the blaze.

According to Peter Knudson of the NTSB, the agency’s investigators arrived Monday and are expected to remain in the area for a few more days as they search for the probable cause of the explosion and fire. They are also looking for any contributing factors.

The NTSB is joined in the investigation by the Arizona Corporation Commission, the U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration, and Kinder Morgan. The Pinal County Sheriff’s Office and the Coolidge Police Department are also involved due to the fatalities.

Knudson told AZ Free News on Thursday that a preliminary report will be posted to the NTSB website in mid-September. However, a final report about the tragedy could take 12 to 24 months to complete.

In a statement released by Kinder Morgan shortly after the incident, the company said it experienced “a pipeline failure” but that the impacted pipeline segment was isolated. It took emergency responders until nearly 8 a.m. to shut off the flow of gas to the area, the Coolidge Fire Department reported.

Coolidge residents miles away from the pipeline reported hearing and feeling the explosion Sunday morning, and a large fireball was visible for miles, including in Casa Grande.

The NTSB is an independent federal agency responsible for determining the probable cause for pipeline incidents, as well as civil aviation, railroad, highway, and marine accidents.