RALEIGH, N.C. (AP) — North Carolina safety regulators said Friday they are investigating 10 companies in the natural gas explosion that killed three people at a Slim Jim plant, including two cited in a lawsuit brought by injured workers.

Labor Department spokesman Neal O'Briant said inspectors are looking at companies including plant owner ConAgra Inc., industrial water heater-maker Energy Systems Analysts in Hickory and mechanical contractor Southern Industrial Contractors in Raleigh.

"Our investigation is focusing around the explosions, but we also will be looking at other aspects of the safety and health program that were not involved in the explosion," O'Briant said.

The June 9 explosion ripped through the plant while 300 people were working, collapsing parts of the roof and crushing nearby parked cars. Nearly 40 people were injured.

Two federal agencies have since blamed natural gas for the blast. Earlier this week, the U.S. Chemical Safety Board said contractors installing a water heater likely vented natural gas inside the building before the explosion as they purged a gas line. The gas should be vented outside.

Southern Industrial attorney Rod Petty said his client wasn't installing the water heater.

"Our scope of work was limited to the gas line tie-in on the roof," Petty said. "We were not involved in purging the gas line in the pump room and we did not connect the gas line to the hot water heaters."

Energy Systems Analysts Inc. is a privately owned Hickory company founded in 1991, according to the company's Web site. A telephone message left at the company after business hours Friday was not immediately returned, and a company lawyer did not return phone messages.

Energy Systems has not been cited or investigated for suspected workplace safety violations in the past, according to the state.

O'Briant said it would be several months before the investigation was completed. Most of the other contractors being probed are temporary worker agencies.

Meanwhile, lawyer David Stradley said his lawsuit on behalf of two injured ConAgra employees had been expanded to included Energy Systems and employee Curtis Ray Poppe, as well as Pasadena, Calif.-based Jacobs Engineering Group.

A listed number for Curtis R. Poppe in Hickory was disconnected. A message seeking comment from Jacobs Engineering wasn't immediately returned.

The attorney said at a news conference it was unclear who ran the gas line into the room where the water heater was installed. He said it also was unclear who hooked the line to the water heater.

"We believe somebody in the parties we have named did this," Stradley said.

Stradley said Energy Systems Analysts and Jacobs Engineering didn't have contractor licenses.

Contractors who install fuel piping are licensed in North Carolina by the State Board of Examiners of Plumbing, Heating, and Fire Sprinkler Contractors. The board's Web site showed no license issued to Curtis Ray Poppe, Energy Systems Analysts or Jacobs Engineering.

UNC Hospitals still were caring for five patients from the Con-Agra blast on Friday. Four were in critical condition and one in fair condition, said Les Lang, a spokesman for UNC Health Care. One patient was discharged Friday, Lang said.