NEW YORK, July 23 -- Horsehead Holding Corp., the largest U.S. zinc producer, said it will continue to produce the metal at a plant in Pennsylvania while investigating an explosion that killed two workers and injured two others.

The blast at the zinc refinery of the Monaca factory occurred at about 4:30 p.m. local time yesterday, Pittsburgh- based Horsehead said today in a statement. Zinc metal production continues at the smelting operations while refining is suspended pending an accident investigation, Horsehead said.

Monaca can refine 90,000 tons of zinc oxide a year and is the country's largest zinc smelter, with the capacity to annually produce 88,000 tons of prime western zinc metal and 15,000 tons of "special special" high-grade zinc metal, according to the company's annual report.

Horsehead fell 36 cents, or 4.4 percent, to $7.73 at 4 p.m. in Nasdaq Stock Market trading. The shares have declined 39 percent this year.

The explosion involved a refinery column that makes zinc oxide, Ali Alavi, a Horsehead spokesman, told the Associated Press yesterday. He didn't return a call today from Bloomberg News seeking comment.

Horsehead said it's working with customers to minimize supply disruptions.

Zinc for delivery in three months on the London Metal Exchange dropped $42.50, or 2.2 percent, to $1,906 a metric ton as of 9:08 p.m. U.K. time.

Horsehead declined to release the names of the employees who were killed or the injured workers who were released from local hospitals yesterday.