On Aug. 15, 2007, a four-man crew cut off a pipeline that feeds the refinery's distillation unit after it caught fire. The contents of the unit burned out within an hour. No one was injured.

On May 6, 2007, a furnace exploded and caught fire, sending a huge plume of black smoke into the air. No one was injured and no homes were evacuated.

In 2005, a fire erupted when a tube carrying hydrogen and diesel fuel ruptured inside a furnace at the refinery. The fire burned for a half hour. No one was injured.

In 2003, asphalt inside a tank exploded and caught fire, again sending a plume of black smoke into the air and causing about $100,000 damage. No one was injured.

Adams said that because no one was injured in those previous incidents, his division was not notified.

"They are required to call us within eight hours if there is a serious injury or there was a fatality," he said.

A team from the U.S. Chemical Safety Board, an independent federal agency, planned to arrive Wednesday to ask about the previous fires.

"We seek to understand why these accidents occur," agency spokesman Daniel Horowitz said. "We also look at the safety culture of the corporation."

The South Davis Metro Fire Agency said Monday's blaze at Silver Eagle was extinguished about 4 a.m. Tuesday, after more than 80 firefighters worked through the night to put it out. An explosion at the refinery occurred about 5:30 p.m. Monday, injuring the four men and sending flames high into the sky. Residents within a half-mile of the refinery were ordered to evacuate to nearby Woods Cross High School. About 100 homes were affected.

"Our concerns were the tank failing or the tanks next to it getting too hot and catching on fire," said Jeff Bassett, deputy fire chief.

Christensen said the explosion appeared to have been caused by fumes escaping from a storage tank's vents. Because the fumes venting from the top of the 13,500-barrel tank were heavier than air, they spread along the ground and across a parking lot, where a furnace inside a small building is believed to have ignited the vapor, he said.

The flames traveled back to the tank, burning off gasoline vapor from the vents. Eventually, firefighters were able to inject foam down the same vents, putting the fire out.

Before the explosion, workers had purged some of the lines connecting the tank, which holds partly refined gasoline. That work is believed to have forced vapors to escape from the tank's vents, and the vapors could have been concentrated enough to catch fire, Christensen said.

In a statement Tuesday, refinery manager Kerry B. Carroll said Bloomfield and McSwain are Silver Eagle employees, and Harter and Benoit worked for Elite Insulation. McSwain, who runs Silver Eagle's refining lab, is a brother of company president Dave McSwain, according to Christensen.

"All four individuals sustained second- and third-degree burns to their upper extremities," Carroll said.

A full investigation into the cause of the fire could take the South Davis Metro Fire Agency a week to complete.

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Contributing: Paul Foy, Associated Press; Ben Winslow

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E-mail: jlee@desnews.com