A massive pipeline explosion that shot a pillar of flame toward the sky after a vehicle drove through a fence and struck an above-ground valve on Monday was not terror-related, authorities said.

Police and local FBI agents investigated and found no preliminary reports that would suggest a coordinated or “terrorist” attack on the liquefied natural gas pipeline, saying “this appears to be an isolated incident,” according to officials in Deer Park.

Sherry and Chad Richard told KTRK-TV that the SUV caught their attention “because the car was moving so slow” before it “just veered off.”

The vehicle went through a fence before striking the valve, leading to a “boom” and “fire everywhere,” Sherry Richard told the station.

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Chad Richard told KTRK that he initially thought the driver suffered a medical emergency because the SUV “drifted” and “really wasn’t going that fast” until getting past the fence.

“My thought was she had a heart attack or whatever and drifted over there because it really wasn’t going that fast,” Chad Richard told KTRK. “But once it got over the fence, it just picked up.”

The explosion incinerated the vehicle and the flames scorched a wide radius, severing adjacent power transmission lines and igniting homes at a distance. Nearly 1,000 homes were evacuated and residents sheltered in schools.

Police did not immediately provide any information about the driver’s identity or condition.

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The tower of flame lit up the sky for more than 12 hours. Deer Park Mayor Jerry Mouton Jr. said the ladder trucks showered houses within its radius from above due to the intense heat.

“A lot of the house structures that are adjacent to that are still catching on fire even though we’re putting a lot of water on them,” Mouton said at an afternoon news conference.

On Tuesday morning, the city of Deer Park said in a statement that the spouting flame had subsided, but the fire continued to burn.

“Progress has been made as first responder crews worked through the night. The fire is significantly smaller,” the statement said.

No volatile organic compounds had been detected, Harris County Pollution Control said in a statement Monday afternoon. The statement said particulate matter from the smoke was moderate and not an immediate risk to healthy people, although “sensitive populations may want to take precautions.” The Texas Commission on Environmental Quality said it was also monitoring the air.

The pipeline’s owner, Dallas-based Energy Transfer, said air monitoring equipment was being set up near the plume of fire and smoke, which could be seen from at least 10 miles away at one point.

The Associated Press contributed to this report.