Nov 20 2008 A gas buildup under the Pueblo Branch Inn was responsible for a Nov. 13 explosion that killed a woman and injured several other people, authorities announced this morning at a news conference in Pueblo.

Investigators from the Bureau of Alcohol, Tobacco and Explosives and the Pueblo Fire Department capped a weeklong investigation by determining that natural gas leaked from a feeder line under the street outside the restaurant, "took the path of least resistance" and ended up collecting in part of the basement, said fire department spokesman Woody Percival said.

The combustible gas/air mix could have been ignited by a compressor motor on a walk-in cooler, pilot lights on several gas-operated appliances or candles that were recently placed atop tables in the dining room - or any number of other ignition sources.

"There are so many (potential) ignition points that we may never know," he said.

Restaurant staff noticed an odd odor before the explosion but assumed it was sewer gas, at one point calling in a drain cleaning company to inspect the drains.

But no one reported smelling the pungent "rotten egg" odor usually associated with natural gas, apparently because an additive that produces the odor - t butyl mercaptan -- was filtered out by soil as the gas traveled to the restaurant.

"It either eliminated the mercaptan, as in filtering it out, or it altered the smell so that it wasn't recognizable to people," Percival said.

The damaged feeder line fed gas into a business across the street.

The ATF, which assisted at the request of Pueblo police, concluded its role Wednesday and will leave Pueblo this afternoon after a final briefing. Pueblo Fire Department investigators will continue investigating what caused the gas pipe to become damaged - an investigation that my lie at the heart of who is legally responsible for the accident.