A contractor clearing a natural gas line during water heater installation likely released a flammable cloud that ignited in a fatal blast at a North Carolina Slim Jim factory, federal investigators said Thursday.

Read Thursday's prepared speech [.pdf]

The U.S. Chemical Safety Board echoed another agency's report from last week in blaming natural gas for the blast that killed three people at the Conagra Inc. plant on June 9.

"A current focus of our investigation is to determine why flammable gas was released into the midst of an occupied building with about 100 people in the immediate vicinity," a statement from the board said.

Board investigator Don Holmstrom said investigators still haven't determined what ignited the gas and wouldn't identify the contractor because the investigation still was being conducted. Access to the area where the blast occurred has been hampered by unstable debris.

Safety board Chairman John Bresland said it was unsafe to purge a natural gas line indoors.

"In my mind that would seem to be a risky operation," Bresland said. "I have never heard of such a thing."

The board said ConAgra was installing a gas-fired water heater that was about 8 feet tall and 4 1/2 feet in diameter in a pump room in the packaging area of the south side of the massive plant. The hot water produced was to be used for cleaning and other purposes.

A new gas line was installed between a rooftop gas main and the heater the week before the explosion. The morning of the blast, a contractor was removing air from the line as the heater was put into service, according to the board statement.

"The CSB is examining the possibility that gases inside the line were likely purged and vented directly into the pump room, in the interior of the building, leading to a flammable gas cloud and an explosion," the board statement said.

Last week, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives also said natural gas was to blame for the explosion.

The board said ammonia gas, which is toxic and flammable, also was in the building and was released in the explosion. The board said it would study the role of ammonia in the accident.

The blast injured 38 workers. A spokeswoman at UNC Hospitals in Chapel Hill said Thursday four people remained in critical condition at the hospital's burn unit while one was in fair condition and one was in good condition.

Investigators started their work at the site last week. But Chairman John Bresland wanted to see the site for himself. He was only able to see the outside of the plant.

CSB investigations usually last nine months to a year. Once the investigation is complete, CSB will make safety recommendations to ConAgra Foods and to regulatory agencies. The board will also hold a public meeting on its report.

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