PITTSBURGH — Federal investigators looking for the cause of an explosion that killed two workers at a zinc plant will be examining similar fatal explosions overseas for answers, officials said Wednesday.

The July 22 explosion at the Horsehead plant about 25 miles northwest of Pittsburgh was similar to two explosions at European plants that used the same technology and killed 11 workers, said Dr. Rafael Moure-Eraso, chairman of the U.S. Chemical Safety Board. Officials couldn't provide details of those blasts but said the process used at those plants and in Western Pennsylvania dates to the 1940s.

The board said it believes last month's blast was caused by a leak in the five-story brick distillation column, where under normal operations natural gas and carbon monoxide are used to oxidize molten zinc as it flows through.

On Tuesday, the board said it would open a full investigation into the explosion. The board doesn't issue fines, but it is empowered to make safety recommendations.

The Pennsylvania explosion killed James Taylor, 53, of Aliquippa and Corey Keller, 41, of Newell, W.Va., who both suffered smoke inhalation.

Lead investigator Roger Evans said the board knows of at least one previous explosion in a distillation column at the Beaver County plant. One employee was injured by flying debris in the 2007 explosion, which was not investigated by the Occupational Safety and Health Administration, he said.

Evans said investigators spent three days at the scene last month and are still collecting documents and information about the most recent and 2007 explosions. He said the plant's owners and the union representing the workers have been fully cooperating.