It may be weeks before owners of International Specialty Alloys learn what caused the explosion at their Neshannock Township plant.

The portion of the plant where the explosion occurred at 9:34 a.m. Wednesday was closed following the blast. Safety engineers at 3 p.m. Thursday said the structure was safe to enter. Following the explosion, there was some question if the structure was safe to enter. Damages caused by the blast had been estimated at $500,000.

“Now we can send people in to clean up and for maintenance,” said Joy Chandler, vice president for corporate relations of Kennemetal, the parent plant. “But we don’t know what caused the explosion. We’ve sent people in to do the root cause analysis.”

Chandler said outside forensic experts who have worked with the corporation before have been called on to help to determine what happened.

The corporation will take advantage of the long Labor Day weekend, she said. Operations, other than cleanup, will be shut down until Tuesday. Raw materials moved out of the structure following the blast can now be returned.

“Maybe by then we’ll have some answers,” Chandler said.

International Specialty Alloys produces product for the aerospace and computer industries.

On Wednesday, processors were adding heat to raw product creating a specialty ingot.

“As the heat was increased, the process foamed and the explosion followed,” she said. “We have been doing that process at that facility for five years. This has never happened before and we don’t know what caused it to happen now.”

Chandler said a similar operation takes place in another portion of the Northgate Industrial Park complex. Although there was no explosion at that part of the operation, it too has been shut down until the company receives answers, she said.

Operations in an adjacent building that the corporation operates in the complex is continuing, Chandler said.

Four employees were injured in the blast.

Jeffrey Grimes of 201 Clen-Moore Blvd., the most seriously injured, was flown to Mercy Hospital in Pittsburgh. Linda Ross of the hospital’s communications department said Grimes remains in serious condition.

The others were treated at Jameson Hospital and released. Their names have not been provided.