FBI (search) agents have ruled out terrorism, but federal regulators estimate it will take them months to determine what caused an oil refinery explosion that killed 15 and injured more than 100.

Investigators from two agencies, the Occupational Safety and Health Administration (search) and the U.S. Chemical Safety Board, arrived Thursday at the 1,200-acre BP plant to start sorting through the debris.

Also Thursday, an FBI spokesman in Houston dismissed a statement posted on an Islamic Web site claiming responsibility for the blast. He said there was no indication of foul play.

Wednesday's blast came during a maintenance period in an area that boosts the octane level of gasoline. An explosion happened during a maintenance period the same time last year, but no one was injured.

"History has shown that many of these kinds of accidents tend to happen before, during or after a maintenance turnaround," said Angela Blair, lead investigator for the Chemical Safety Board.

About 1,100 employees and 2,200 contract workers were on site when the explosion took place, shortly before 1:30 p.m. Those killed were all contract workers. It was not immediately known who employed four, but 11 worked for J.E. Merit Constructors Inc., a subsidiary of Jacobs Engineering Group Inc. in Pasadena, Calif.

Jacobs CEO Noel Watson said Jacobs was not performing any work on the unit that exploded, but "a number of our employees were meeting in a staff office trailer about 150 yards from that unit."

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, Blair said.

"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. The blast would not affect the U.S. gas supply, BP officials said.

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

Of the injured Wednesday, many were seriously burned and suffered broken bones.

"There are several patients who are literally fighting for their lives," said Dr. Bruce Zachariah of the University of Texas Medical Branch in Galveston, where eight of the 18 workers still there are listed in critical condition. He said some face multiple surgeries, skin grafts and possibly amputations.

C.O. Magee, a Presbyterian minister, was trying to help mourning families deal with 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. "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.