INDIANAPOLIS -- Investigators in the Indianapolis explosion that killed two people and decimated a neighborhood believe natural gas was involved and are looking at appliances as they search for a cause, a state official said Tuesday.
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.