Erickson said more gas blasts are caused by appliances than by pipelines, but even those are rare. Technological advances such as microprocessors and the switch from pilot lights to electronic ignition have made appliances safer, he said. Gas companies have been required since 1970 to add a chemical that smells like rotten eggs to the odorless gas to make leaks easier to detect.
Erickson said a temperature sensor on furnaces is supposed to prevent a gas valve from opening if the pilot light is out. But if that device were to malfunction -- or if a gas pipe in the house were to break -- it could allow a significant gas buildup and cause a big explosion, he said.
"To get a house to fill up with gas would take a pretty major leak. It would be more than just a pilot light that went out and the gas continued to flow," Erickson said.
He said that between 60 and 80 cubic feet of natural gas could flow out of a defective furnace or broken pipe every hour, rapidly filling a home or building.
The head of a company that does furnace repairs in Indianapolis said the blast's size made it unlikely that it had been caused by a leaking appliance.
"One hell of a lot of gas had to be leaking out ... and that's typically not symptomatic of a furnace problem," said Sergei Traycoff, president of Bolls Heating and Cooling. "I've never heard of one causing this big a blast."
Consumers can best protect themselves by having their furnaces inspected regularly, he said.
Erickson said it was odd that the blast apparently flattened two homes side by side. Generally, if a house explodes, it will knock out the wall of the home next door, but not level it, he said. For that to occur, both homes would virtually have to have gas leaks that ignited at the same time, he said.
Schreiber said gas explosions create an intense wave of heat that can ignite surrounding homes.
"It goes very quickly. It's just a 'whoosh,' you know, like if you have a gas stove or a grill where it doesn't ignite immediately and there is a whoosh sound. That's kind of what happens here, but at a much, much greater level. It's a quick event, not a lingering thing."
Glenn Olvey, 52, isn't sure what caused the blast that wrecked his home and vehicles. But he knows his family is fortunate to be alive. The blast hurled Olvey several feet and trapped him, his wife and one of their two teenage daughters when their roof collapsed.
"I have been through car accidents, I've been through motorcycle accidents, I've been through tornadoes. I have never, never had anything like that."
Olvey said he and his wife, Gloria, have struggled knowing that the explosion killed Jennifer and John Longworth, just two doors down.
"We're alive for a reason. ... What it is, I don't know."