Crews examine the damage Wednesday, April 22, 2026, after two homes exploded on the 15000 block of Preston Hollow Drive on Tuesday evening, April 21, 2026.
Katina Zentz/San Antonio Express-News
As federal investigators prepare to release a preliminary report on two house explosions that injured five residents on the North Side, the former director of the agency is weighing in with a potential theory.
Robert Hall, who led 171 investigations during his 12-year tenure at the National Transportation Safety Board, told the San Antonio Express-News he was struck by one possible explanation when he saw video of crews removing an oak tree near the homes.
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“Likely a tree root impacted a pipe and caused a failure of the pipe in some way,” Hall said. “There is something that can happen with plastic pipes when they’re impinged and put under stress. There’s a phenomena called slow crack growth, so you can actually develop a crack in the pipe.”
The two houses in the 15000 block of Preston Hollow Drive, separated by one home in between, exploded the evening of April 21 less than three hours apart, injuring four adults and one teen.
Three days later, KENS-TV captured crews removing the tree and probing the roots it left behind in the dig site. Residents told the station they believed investigators’ “working theory” is that the roots caused the gas leak.
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Authorities have not released details about the material or condition of the pipelines involved. Those are facts that will likely come in a preliminary report expected by May 21 from the NTSB, which is leading the investigation.
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Three residents were injured in the first explosion: MacArthur High School math teacher Kimberly Nowell, Wayside Chapel student pastor Tim Nowell and their teen daughter Ali.
Brooke Army Medical Center listed Kimberly Nowell in serious condition Friday and Tim Nowell in critical condition. A spokesperson said Ali was not being treated there. Church representatives said in the days following the explosions that she was in stable condition.
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Two residents injured in the second explosion are also being treated at Brooke Army Medical Center. Jose Ochoa was in good condition Friday and Mayte Reeves was in fair condition.
What is slow crack growth?
The homes on Preston Hollow Drive were built in 1993, real estate listings show. Officials first took notice of slow crack growth, then known as as “brittle-like cracking,” that same decade.
Plastic pipelines soared in popularity for their price and efficiency starting in the 1960s, with the NTSB estimating that hundreds of thousands of miles of plastic piping were installed by the ’90s.
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But an NTSB investigation published in 1998 found that “the material may be susceptible to brittle-like cracking under conditions of stress intensification.” At the time, experts believed it was the second-most frequent reason for pipe failure, second only to excavation damage.
“Until then, they didn’t identify the material problems that were causing it,” Hall said.
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The special investigation was prompted by several accidents, including one in Texas. In August 1997, an explosion killed one person in Lake Dallas after metal piping pressing against a plastic pipe caused a crack. The case garnered new attention this year when another home exploded a half-mile away.
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There is no one factor that leads to slow crack growth, according to Hall. A variety of circumstances can influence stress on the pipe, including ground temperature, soil environment and installation practices.
Other materials are prone to different problems, he said. For example, steel piping can be affected by corrosion or welding issues.
District 10 City Council Member Marc Whyte, whose district includes the Preston Hollow neighborhood, has blamed aging infrastructure for the Preston Hollow explosions.
He and two other council members also filed a memo last week requesting a special session in May to review the effectiveness of the emergency response.
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Residents have questioned why they were not evacuated after the first explosion, but Hall said it’s not atypical, especially if there was no odor of gas. The soil could have masked the scent depending on the depth of the pipes.
“There have been other cases where there has not been an evacuation because there may not have been a thorough investigation to determine the safety of the area,” Hall said.
One former NTSB investigator, Ravi Chhatre, said that he thought the delayed evacuation was unusual. He investigated about 20 cases during his career and could only recall one similar case.
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“I was a little bit surprised,” he said.
At a CPS Energy Board of Trustees meeting April 27, the first since the explosions, President and CEO Rudy Garza assured the public that the utility is doing “everything we can” to support victims and customers.
Crews distributed gas and carbon monoxide alarms to residents Monday with help from the San Antonio Fire Department, the utility said in a statement. Anyone who smells gas should leave the building immediately without investigating where the odor is coming from.
“Typical human nature when you smell gas is, we want to investigate and find out where the odor is coming from,” Chhatre said. “Then you turn the light switch on. That’s a big ‘no no,’ because you have no idea that light switch may cause the explosion.”
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Once the NTSB concludes its investigation, which can take anywhere from a year to two years, the agency will release the probable cause of the accident. It will issue relevant safety recommendations, which can cover everything from state regulations and oversight to operator standards and first responder protocols.
“The idea really is not to blame anybody, the idea is to find the cause of the accident,” Chhatre said. “Nobody can turn the clock backwards, but NTSB’s goal is to prevent similar future accidents.”