INDIANA, Pa. (AP) — Investigators searched a western Pennsylvania tire plant Thursday, a day after a reported explosion and fire in cold storage area of what a company official said is the oldest tire manufacturing plant in North America.

The fire at Specialty Tires of America in Indiana, Pa., was reported about 5 p.m. Wednesday, and injured three firefighters. Three workers who tried to extinguish the blaze before firefighters arrived were treated and released for smoke inhalation, Mark Grant, the company's vice president and general manager told The Associated Press on Thursday.

Grant and Trooper Timothy Frew, the fire marshal, said the fire began in an attached building that is used for cold storage of tire making materials. Witnesses reported hearing an explosion, but it was unclear if that's what caused the fire, or whether the explosion resulted from the flames that produced dark grey and black smoke that could be seen for miles.

The plant opened in 1950 and, according to Grant, is the "oldest tire manufacturing plant in North America." The fire was contained to the cold storage room which was added in 1997 and is separated from the rest of the plant by large steel doors, Grant said.

"It's actually just a walk-in cooler" about 12-by-24 feet, Grant said. "It's used to control the temperature and humidity of some materials we use to manufacture tires." The room contained steel-belting material used to make radial tires.

Frew did not immediately return a call for comment Thursday, but said in an earlier news release he couldn't immediately determine what caused the fire.

Grant said no finished tires or chemicals, which could have greatly fueled the blaze, burned.

Only about 30 employees were at the plant when the fire broke out, because the factory is in the midst of a yearly maintenance shutdown that lasts a week, Grant said. Typically, about 100 employees would be in the factory on any given shift. Although the fire destroyed the cold storage area, the plant maintenance work was continuing Thursday and it was still scheduled to resume normal production next week.

Specialty Tires has about 370 employees and manufactures private label tires, as well as its own brand names of tires for specialty uses, including American, American Farmer, American Racer and American Miner tires. The plant makes about 600,000 tires a year for aircraft, race cars and all-terrain vehicles, farm equipment and antique cars.

The employees who suffered smoke inhalation were back to work Thursday, Grant said.

Indiana Fire Chief Chuck Kelly told The Indiana Gazette that the three firefighters injured included one who was treated for facial burns and another for heat exhaustion. A third suffered either a broken or sprained ankle.

The plant is about 45 miles northeast of Pittsburgh.