Indianapolis Homeland Security Director Gary Coons made the announcement after the National Transportation Safety Board said investigators had found no leaks in the gas main or pipes leading into the house that exploded. The explosion Saturday leveled two homes and left dozens more uninhabitable.

Coons said his "investigators believe natural gas is involved" and were "recovering the appliances from destroyed homes to help determine the cause."

"Based on the NTSB statement, our focus is on the houses and appliances," Coons said in a statement.

The blast showed some signs that aren't typical of a natural gas explosion caused by an appliance, experts said, but it still could have been tied to a faulty furnace -- if conditions were right.

An owner of the house believed to be at the center of the explosion has said the home's furnace had been having problems. But experts said that doesn't mean homeowners should be worried that their furnaces are about to explode.

John Erickson, vice president of the American Public Gas Association, said it would take a far more serious malfunction than just a pilot light going out.

The natural gas lines inside the house would be under the oversight of the utility or the state, said NTSB spokesman Keith Holloway.

Citizens Energy spokeswoman Sarah Holsapple said the utility had found no leaks in its underground facilities in the neighborhood.

The Indiana Utility Regulatory Commission had no comment Tuesday.

Richard Schreiber, a forensic mechanical engineer with Intertek AIM in Sunnyvale, Calif., said it's usually immediately evident whether the blast was caused by natural gas.

Schreiber said that with explosions involving solids such as dynamite, the center of the blast is tightly concentrated, creating a crater. Explosions caused by flammable gas are typically spread out over a wide area, such as throughout the interior of a building filled with leaking gas, he said.

"If the investigators don't find a crater, that pretty much means it was something other than a solid phase explosion," he said, meaning it's likely to be a gas explosion.

But he also said such investigations can still take time.

More than a dozen home explosions linked to natural gas have occurred in the last two years. Many involved a single home, though more devastating blasts tied to pipelines -- including a 2011 explosion in Allentown, Pa., that killed five people and a blast in 2010 in San Bruno, Calif., that killed eight people and destroyed 38 homes -- have been reported. A gas leak in a Colorado home last month sparked an explosion that sent five people to a hospital and damaged several homes.

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."