Federal investigators are looking for the cause of an explosion that killed two zinc plant workers. They will also be looking at similar fatal explosions overseas for answers.

Officials with the U.S. Chemical Safety Board say the July 22 explosion at the Horsehead Corp. plant in Monaca was similar to explosions in Europe that used the same technology and killed 11 workers. Officials say the process dates to the 1940s.

The board says it believes last month’s blast was caused by some sort of leak in the distillation column, but its investigation is still in its infancy. When operating normally, natural gas and carbon monoxide are used to oxidize molten zinc as it flows through the column.

The board has said it would open a full investigation into the explosion.