A refinery explosion that severely damaged 10 homes this week was caused by a burst pipe that sprayed hydrogen gas onto a nearby heater, the U.S. Chemical Safety Board's lead investigator said.

Donald Holmstrom said investigators don't know why the pipe suffered what he called a "catastrophic failure." He said the safety board will test what's left of the pipe to determine what went wrong.

"I can assure the people who live around this area that we will do a thorough investigation," Holmstrom said.

The 10-inch pipe was carrying 630 pounds of pressurized hydrogen when it ruptured Wednesday morning.

The explosion at the Silver Eagle Refinery smashed windows, bent garage doors and peeled siding from 10 nearby houses. No injuries were reported, but Holmstrom said five workers were inside the unit just minutes before the blast.

"There was a large fireball in the unit," Holmstrom told reporters Saturday. "The unit was immediately engulfed in flames that went as high as 100 feet in the air. We're extremely fortunate that no one was hurt."

Holmstrom says the failure occurred near the elbow on the horizontal section and spewed the 800-degree hydrogen east toward a nearby neighborhood.

The pipe was attached to a reactor that removes waxes from diesel fuel. The pipe normally carries diesel and hydrogen, but the refinery was performing maintenance at the time of the failure and only hydrogen was in the pipe, Holmstrom said.

The refinery in Woods Cross, about five miles north of Salt Lake City, also had fires in 2003, 2005 and 2007, according to federal records. The board also was looking into an Oct. 21 fire at a nearby Tesoro Corp. refinery.