A safety inspector said Tuesday it was "a miracle" the catastrophic failure at a Woods Cross refinery, believed caused by worn-out piping, did not result in "more serious consequences."

Video captured by a surveillance camera at the Silver Eagle Refinery on Nov. 4 shows a pick-up truck pass the processing unit seconds before leaking hydrogen gas erupted in a fireball that extended 100 feet into the air.

Moments earlier, a worker had taken readings next to the 10-inch pipe, which ruptured with such force it bent around an adjacent steel support beam. Four other employees working nearby when the pipe blew were knocked to the ground but not seriously injured.

Investigators also have learned a FrontRunner train moved through the area shortly before the blast.

"Clearly, this explosion had the potential to cause deaths or serious injuries had it occurred even a few minutes earlier or later in the day," said Don Holmstrom, an investigations supervisor for the U.S. Chemical Safety Board.

The board held a news conference at a hotel not far from the Silver Eagle Refinery to discuss preliminary findings of its investigation, which is pinpointing a "mechanical integrity program that had serious deficiencies."

Silver Eagle agreed Friday to shut down temporarily to correct what John Bresland, U.S. Chemical Safety Board chairman, characterized as "potentially serious safety problems." Four of its five units are off, and the last should be shut down within days.

Bresland said it was the first time the board has asked a chemical plant or refinery to shutter operations because an investigation found widespread problems.

The explosion at Silver Eagle damaged more than 100 homes in a subdivision separated by only train tracks and a fence from the refinery.

Bresland toured the neighborhood Tuesday and was "struck by the seriousness of the damage." Four homes have been declared uninhabitable, he said.

As part of its investigation, the board will "seek to determine how residential housing came to be located in an area where it could be affected by potential refinery explosions," Bresland said. "We will be examining whether existing guidelines for the siting of hazardous facilities provide enough protection for residents."

Holmstrom said the board's investigation shows that a 2007 inspection of the failed pipe calculated its thickness at one-half inch -- four times higher than the one-eighth inch thickness measured after the explosion.

He said a likely explanation for the discrepancy was a miscalculation of the thickness of the pipe during the 2007 inspection.Investigators also have found "a significant percentage of the pipes and vessels have no documented thickness readings at all," Holmstrom said.

Refinery managers now acknowledge that values for equipment throughout the plant have been miscalculated, he said. There is the possibility that "multiple pieces of equipment have been operating at below the required thickness for safety and creating the potential for other serious accidents," Holmstrom said.

He said Silver Eagle replaced the private contractor who made the 2007 readings after questions were raised about the validity of its work. With a new contractor in place, it was rechecking the plant's equipment when the blast occurred, he said.

Silver Eagle officials issued a statement Tuesday defending its operations, saying it has followed "an established mechanical integrity program and disagrees with any suggestion or assertion to the contrary."

"Silver Eagle has also worked hard in recent years to upgrade the refinery and improve safety," the company said. "However, in light of the November 4 incident and the preliminary findings, it is apparent to the refinery that the program was insufficient and that changes need to be made."

It has hired an internationally respected consulting firm to assist in that effort, the company said.