People walk away from the United States Steel Corp.'s Clairton Coke Works in Clairton Wednesday where 15 people were injured when a coke oven exploded around 9:30 a.m. Wednesday. A cause has not yet been found.

PITTSBURGH — From the outside, a typical coke plant looks like something from a dystopian science-fiction movie: a maze of metal buildings, pipes, conveyer belts and tall smokestacks belching white puffs of smoke and steam into the air.

Inside, there's danger seen and unseen: ovens that heat coal to more than 3,000 degrees, massive steel doors for loading the ovens, moving cars and chutes for coal, and gases that can cause cancer or ignite.

Turning coal into coke, a raw material used in steelmaking, is a complicated and dangerous process, as evidenced by the explosion Wednesday that injured 20 people at the country's largest coke plant. But those familiar with the industry say it can be done safely.

"When people ask me, is this a safe place or is this a dangerous place? (I say) it's both," said Michael Wright, head of the health, safety and environment department for the United Steelworkers union. "Our philosophy is that anything can be done safely if you work at it, but clearly something went wrong here."

The Occupational Safety and Health Administration was investigating the blast at U.S. Steel's Clairton Coke Works. A cause was not yet known.

The plant's largest battery, a structure containing dozens of coke ovens, was shut down after the blast. Company officials said it was operating at a reduced rate Thursday and expected it to be fully operational in a week. The rest of the plant was operating normally.

Fourteen employees and six contractors were injured in the blast. Nine remained hospitalized Thursday, including two in critical condition at West Penn Hospital and three in critical condition at UPMC Mercy.

About 1,500 people work at the plant, one of four operating coke plants in Pennsylvania and 20 in the U.S.

Clairton, a town outside Pittsburgh of about 7,800 residents and with a median income of about $26,000, is perhaps best known as the setting for "The Deer Hunter," the 1978 Vietnam epic that won five Oscars.

It's not clear how many residents work at the plant, which dominates the city's landscape and has been owned by U.S. Steel for a century. Census figures show that about 12 percent of people in Clairton work in manufacturing.

To make coke, coal is baked in special ovens for hours at high temperatures to remove impurities that could otherwise weaken steel. The process creates what's known as coke gas — made up of a lethal mix of methane, carbon dioxide and carbon monoxide.

Because of those and other hazards, coke plants must meet federal requirements on everything from how machinery operates to workers' protective gear. Employees have flame-retardant clothing, hardhats, safety boots and respirators, depending on their job.

Clairton was last inspected by OSHA in 2009, and the agency found no violations. A U.S. Steel spokeswoman said there was no one available Thursday to talk about working conditions at the plant.

Some coke plant accidents have been deadly. Online OSHA records show, among other incidents, that a worker at an AK Steel Corp. plant was crushed between two machines and killed in 2006.

Another explosion happened at Clairton in September 2009, when a maintenance worker was killed. The blast happened in a different area from Wednesday's blast, and OSHA has issued no citations in that case.