"Deeply disturbing" staff errors led to the oil refinery explosion and fire that killed 15 workers, and some employees could be dismissed as a result, plant operators said Tuesday.

BP PLC (search), one of the world's largest oil companies, released its interim report on the March 23 blast at the Texas City plant, which happened in a unit that boosts the octane level of gasoline. More than 170 workers were injured.

"The mistakes made during the startup of this unit were surprising and deeply disturbing," Ross Pillari (search), president of BP Products North America, said in a statement. "The result was an extraordinary tragedy we didn't foresee."

BP, formerly British Petroleum, said it was meeting with blast victims' families and attorneys to begin to settle claims against the company.

Supervisors and hourly workers face discipline ranging from written reprimands to dismissal, Pillari said. He declined to say how many employees would be punished.

The BP investigation determined that fluid level in a tower was 20 times higher than it should have been. Water or nitrogen in the tower when the unit was restarted may have caused a sudden increase in pressure that forced hydrocarbon liquid and vapor into the unit's stack.

But investigators still don't know what ignited the resulting vapor cloud. Earlier theories have suggested that sparks from a running truck engine could have been to blame.

Investigators found that supervisors seemed to be absent at times during the unit startup, and crews didn't know who was in charge.

Also, any of six supervisors had a six-minute window in which they could have sounded an alarm to evacuate the area, but that alarm was never sounded, Pillari said. The decision, he said, denied other workers "the opportunity to get out of harm's way."

The U.S. Chemical Safety Board (search) said it would consider BP's findings as part of its ongoing investigation.

Initial reports after the explosion put the number of injured closer to 100, but the company raised that number Tuesday to more than 170.

The BP refinery processes 433,000 barrels of crude oil a day and 3 percent of the nation's gasoline. The blast was the plant's third accident in a year, following a March 2004 explosion that caused an evacuation and a September accident in which two workers were fatally burned by superheated water.

The March blast was the deadliest 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.

Texas City (search) 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 an explosion that killed 576 people.