PITTSBURGH (AP) - The U.S. Chemical Safety Board says a 2010 explosion that killed two workers at a former Horsehead Corp. zinc plant in western Pennsylvania was caused by recurring problems with a sump system meant to drain the volatile chemical from a distillation column.

The report says a history of blockages and smaller explosions at the Potter Township plant caused hazardous conditions to be "normalized" before the fatal blast on July 22, 2010.

Fifty-three-year-old James Taylor, of Aliquippa, and 41-year-old Corey Keller, of Newell, West Virginia, both died of smoke inhalation from the flash fire and explosion that resulted.

A Horsehead spokesman didn't immediately comment on the report issued Wednesday, though the company has filed objections to the findings saying the scientist who issued the report didn't visit the site or personally inspect the debris.

Contacted Thursday afternoon, Ali Alavi, Vice President-Corporate Administration, General Counsel and Secretary of Horsehead Corporation, said the company still owns the property, but the plant has been demolished.