It is still not clear what caused an explosion at the Babine Forest Products sawmill in Burns Lake, B.C., in January that killed two men, according to a preliminary update issued by WorkSafeBC on Wednesday.

"We still don't have the cause of this catastrophic explosion," said spokeswoman Roberta Ellis, who described the investigation as one of the largest every undertaken by WorkSafeBC.

Sawdust is just one of many possible factors under investigation, according to Ellis. Other possible factors include the use of pine beetle kill wood, the weather, and possible ignition sources such as natural gas and propane.

A U.S. lab has been hired to specifically investigate the role sawdust may have played in the explosion and the final investigation is expected to take another two to three months, she said.

Outside experts have pointed to a build up of extremely dry dust from pine beetle wood as the possible cause of the Burns Lake explosion in January and of the explosion at the Lakeland sawmill in Prince George last month that also killed two people.

Following the second explosion, WorkSafeBC ordered clean-ups and inspections at all B.C. mills to ensure sawdust is being properly dealt with.