The death toll in a thunderous explosion at a BP refinery (search) climbed to 15 Thursday as investigators tried to determine the cause of the worst accident in the nation's gas and chemical industry in nearly 15 years.

A worker who was thought to have checked out and left the refinery was instead found dead near the site of the fiery blast, BP spokesman Bill Stephens said.

More than 100 people were injured Wednesday in the explosion, which plant manager Don Parus said happened during maintenance work in an area of the refinery that boosts the octane level of gasoline.

Another explosion at the same plant happened during the same maintenance period almost exactly one year ago. There were no injuries in that blast.

"History has shown that many of these kinds of accidents tend to happen before, during or after a maintenance turnaround," said Angela Blair, the U.S. Chemical Safety Board's (search) lead investigator.

Refineries throughout the United States commonly use contract employees during so-called turnaround periods, when routine maintenance is completed, she said. Most refineries operate around the clock for 18 months to five years before taking equipment down for repairs, she said.

About 1,100 employees and 2,200 contract workers were at the refinery when the blast shot flames into the sky, forced schoolchildren to cower under desks and showered the plant grounds with ash and blackened metal. It rattled windows more than five miles from the 1,200-acre plant near Houston.

Fifteen contract workers were killed. J.E. Merit Constructors Inc. (search), a subsidiary of Jacobs Engineering Group Inc. in Pasadena, Calif., said it lost 11 workers. It was not immediately known who the other victims worked for.

Investigators from the Occupational Safety and Health Administration (search) and chemical safety board arrived Thursday.

A union official complained that hiring contractors is a cheap method that leads to safety shortcuts and tragedies like Wednesday's.

"They want that unit up and running by that date at all costs," said Allan R. Jamail, a representative of the United Association of Journeymen and Apprentices of the Plumbing and Pipefitting Industry.

John Browne, CEO of the BP Group, insisted safety is the first priority: "We don't produce day-to-day just to make a quick buck irresponsibly. We just don't do that kind of thing."

Al Tribble, an FBI spokesman in Houston, said the FBI ruled out terrorism and that it does not appear there was any foul play. Making such determinations have become part of accident investigations since the Sept. 11 attacks.

He also dismissed a statement posted on an Islamic Web site claiming responsibility for the blast.

"It's clear that we have a lot of work to do in the coming days to make sure we understand exactly what happened, and we're going to do that," BP America President Ross Pillari said. "We are going to put all of our resources into it."

The plant processes 433,000 barrels of crude oil a day, producing 3 percent of the U.S. gasoline supply. Other than the unit affected by the blast, the rest of the refinery was running normally, Stephens said. Browne said the blast would not affect the U.S. gas supply.

It was the deadliest accident in the nation's gas and chemical industry since an explosion at an Arco Chemical Co. plant in nearby Channelview killed 17 people in 1990.

C.O. Magee, a Presbyterian minister, said he was helping mourning families and calming their anger, but sometimes all he could do was listen.

"You're caught up in the grief. You're caught up in this disaster," he said, a tear sliding down his face. "At times you feel sort of helpless, but it's just being there. It's being there to listen."

The BP plant and Texas City, population 40,000, have dealt with two other recent refinery accidents.

OSHA fined the refinery nearly $110,000 after two employees were burned to death by superheated water in September.

Another explosion forced the evacuation of the plant for several hours last March. Afterward, OSHA fined the refinery $63,000 for 14 safety violations, including problems with its emergency shutdown system and employee training.

Texas City is the site of the worst industrial accident in U.S. history. In 1947, a fire aboard a ship at the Texas City docks triggered a huge explosion that killed 576 people and left fires burning in the city for days.