PITTSBURGH — An explosion that killed two welders and caught a natural gas well on fire last month was triggered by flammable fumes from a relatively small amount of oil remaining in an aboveground storage tank, a fire marshal said.

Allegheny County Fire Marshal Robert Full said Monday he's "confident" that's what caused the July 23 explosion at a well near Pittsburgh owned by Huntley & Huntley Inc., of Monroeville. The men killed worked for Northeast Energy Management Inc., of Indiana, Pa., which Huntley & Huntley had hired to weld leaks in the tank.

The well produced methane gas but also drew oil out of the ground as part of that process, Full said. The leaky tank was used to store that oil and had been almost emptied before the welders arrived. The explosion damaged the gas well and caused it to catch fire, Full said.

The metal tank could hold about 200 barrels of oil, or roughly 8,400 gallons, and all but about 200 gallons had been removed. The valve used to empty the tank was about a foot off the ground, so the only oil in it was that below the valve, Full said.

Because it was hot that day, with temperatures in the 90s and a heat index near 100, the metal tank heated up, causing oil vapor to fill the tank, Full said. The welders, who were there to repair pinhole leaks in the tank, were killed when a spark ignited that vapor, causing the five-ton tank to rocket over 100-foot-tall trees and land in the woods 220 feet away, chief deputy fire marshal Donald Brucker said.

Brucker said findings that the tank still had some oil in it might result in updated safety procedures for maintaining such tanks.

The federal Occupational Safety and Health Administration routinely investigates workplace fatalities. Spokeswoman Lenore Uddyback-Fortson said the agency doesn't comment on preliminary findings and won't have anything to say until its investigation is finished.