More than 12 hours after an explosion that "sounded like a jet engine going 5 feet above your house," as one neighbor put it, the fire, fueled by the well's gas, continued to shoot flames and smoke into the air, causing a hissing sound that could be heard a quarter-mile away.

The heat from the blaze -- which caused a tanker truck on site that was full of propane gas to explode -- was so intense that first responders from local fire departments had to pull back rather than risk injury.

"They essentially retreated to let the fire burn," said John Poister, spokesman for the Pennsylvania Department of Environmental Protection, which had three people on site investigating.

State police said they were told it could take days to contain the fire.

"We're being told ... the site itself, that fire, will not be contained and we will not have access to that property for at least a few days," Trooper Stefani Plume said at a news conference Tuesday.

Experts on well fires like this were flown in Tuesday from Houston.

Patti Green, a spokeswoman for Wild Well Control, the company Chevron called in to try to contain the blaze, said it would not be unusual for a response team to let a fire burn before making an attempt to knock it down.

The question that remained unanswered Tuesday was what caused the explosion.

Though the fire was initially thought to be a "blowout" in which there was loss of control at the well head during drilling that resulted in a release of natural gas, Mr. Poister said he has been told that it was not a drilling-related accident.

Instead, he said, the well had long since been drilled and crews were on site early Tuesday morning putting in pipe that would connect the well to Chevron's gas-gathering network -- the final stage before the well goes into production.

DEP records show that Chevron's Lanco 7H well was drilled in March 2012 -- as were two other wells on the same well pad -- and had not yet begun to produce gas.

DEP's online records also show the state had not issued any violations against Chevron for any problems related to the drilling of the three wells on the well pad.

In December, Chevron was given one violation for an incident related to the well site -- for failure to comply with the terms and conditions of the state's site permit -- but no details of that violation were immediately available.

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