A fire marshal says an explosion that killed two welders and caught a western Pennsylvania natural gas well on fire last month was triggered by flammable fumes from oil in a storage tank.

Allegheny County Fire Marshal Robert Full says he's confident a welder's spark ignited the vapor and caused the July 23 explosion at a well near Pittsburgh owned by Monroeville-based Huntley & Huntley Inc. The welders worked for Indiana, Pa.-based Northeast Energy Management Inc., which was hired to fix leaks in the tank.

The well produced methane gas but also drew oil from the ground. The leaky tank was used to store that oil and had been emptied of all but a couple hundred gallons before the welders arrived.

The two companies haven't responded to requests for comment left Monday.