BP said it could not immediately pinpoint the cause of the blast at the giant 470,000-barrel-per-day (bpd) Texas City facility, the third largest in the United States.
Terrorism was ruled out early on.
"We have no reason to believe this was anything caused by an outside agent," said company spokesman Hugh Depland.
Another blast rocked the same refinery on March 30, 2004, when a large explosion and fire occurred in a gasoline-making unit â€” but there were no injuries.
That accident resulted in citations for 14 alleged violations from OSHA. OSHA immediately began in an investigation into the latest incident, BP, headquartered in London, said.
Plant spokesman Bill Stephens said more than 70 of the injured had been working in the refinery at the time of the explosion at 1:20 p.m. local time. Others were outside the complex and hurt by flying glass and falling ceiling tiles.
The blaze was extinguished after two hours.
Plant employee John Yarbor told a local television station he was 90 feet from the unit when it exploded. "It literally lifted you up and slammed you to the ground," he said. "I could feel the heat on me and just ran." He escaped injury.
Rose Martin, who works near the refinery, said: "It shook everything. As soon as I walked out the door, it was nothing but fire and black smoke."
Rescue teams combed through piles of rubble and the remains of shattered buildings in the plant to search for the dead and injured.
BP took over the plant, which first began operations in 1934, when it bought U.S. company Amoco in 1999.
Texas City, 35 miles southeast of Houston, is home to several petrochemical and refining plants and no stranger to catastrophe.
In April 1947, Texas City was the site of one of the worst industrial accidents in the United States when a ship full of fertilizer component ammonium nitrate blew up, killing as many as 800 and injuring an estimated 5,000.
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