A lawsuit filed by victims of the ConAgra plant explosion expanded to place blame on two more companies.

The explosion, which federal investigators believe was caused by a natural gas leak, killed three workers June 9.

Attorney David Stradley represents two workers who were injured and are seeking at least $10,000 in damages plus workers compensation benefits.

Stradley said he believes the two companies, Energy Systems Analysts and Jacobs Engineering Group, along with the first company to be named, Southern Industrial Constructors, were all negligent and played a role in causing the explosion.

Stradley said all three companies were involved in hooking up a natural gas line that was being used for a new water heater in the pump room where the explosion happened.

The lawsuit claims the line leaked natural gas into the room and no one in the plant was evacuated.

"I would characterize it as more than human error. I would characterize it as human recklessness at best," Stradley said.

Stradley said he believes Energy Systems Analysts and Jacobs Engineering Group were not licensed to perform work with natural gas lines.

The lawsuit also named an employee at each company who was allegedly overseeing some of the work.

In a statement released this week, Southern Industrial Constructors said "it does not appear that the work performed by Southern Industrial was in any way involved in the explosion."

Calls late Friday to the other companies were not returned.

Stradley said he anticipates more workers will join the lawsuit, and that it's possible more companies will be named as defendants.

The state Department of Labor said Friday it was investigating ten companies that it said had employees on site at the ConAgra plant at the time of the blast.