Welders using torches against safety guidelines ignited explosions that demolished a rocket fuel plant, killing two people and causing shortages of a chemical vital to the nation's defense and space programs, investigators said.

Clark County Fire Chief Roy Parrish announced Thursday that the cause of the May 4 disaster was spewing molten iron from an oxy-acetylene torch used to install paneling at Pacific Engineering and Production Co. (PEPCON) in nearby Henderson, one of only two plants in the nation that manufactures ammonium perchlorate.The explosions injured more than 350 people, damaged more than 3,000 buildings in the Las Vegas Valley and sparked political and public controversies over safety standards and industrial zoning.

The damage has been estimated at $73 million, and destruction of the plant caused a critical national shortage of ammonium perchlorate, the oxidizing agent of solid rocket fuel used to power the nation's ICBMs and the space shuttle.

The blaze, punctuated by several small explosions, destroyed 8.5 million pounds of ammonium perchlorate in less than five minutes, said investigator Capt. Bob James. He said two of the explosions measured 3 and 3.5 on the Richter scale used to measure earthquakes.

Parrish said two welders preparing to hang new siding on the west side of the building were working without a fire watch, a backup team capable of putting out a blaze, when sparks from the torch set fire to building materials, scattered ammonium perchlorate, plastic drums and manufacturing equipment.

Parrish said statements by the welders, both employed by PEPCON, helped investigators trace the cause of the disaster. He said the fire started by the welding equipment ignited nearby debris and then quickly spread to the giant storage fields several hundred feet away.

Parrish's brief technical statement released Thursday contradicted PEPCON's official position that the explosion was caused by leaks in a large natural gas line that runs under the property.

"Natural gas was not involved in the initial cause and origin," Parrish told a news conference.

But Keith Rooker, executive vice president and general counsel for PEPCON, challenged the accuracy of the Fire Department report, saying nothing in it would change the company's opinion that large volumes of natural gas were involved in the disaster.

"Nothing in the Clark County Fire Department report or discovered in the independent professional investigation to date causes Pacific Engineering to modify its position as previously stated on a number of occasions respecting the presence of large volumes of natural gas without which the massive fire and explosions would not have occurred," said a statement released by Rooker.

Owners of PEPCON are considering three sites on which to rebuild the plant - one near Cedar City, another about 40 miles from Las Vegas; and another on a southern Nevada Indian reservation where the operation would be exempt from state and county safety inspections.