"Extensive gas explosions are not easy to put back together," forensics consultant Jay A.Siegel said. "Finding the pieces and putting them back together is a giant puzzle."
Randall Cable, an attorney for homeowner Monserrate Shirley, said Wednesday that the woman's daughter had complained of an odor outdoors and in the garage area for several weeks before Saturday's blast that leveled two homes and left dozens more uninhabitable. Two people were killed.
"Once they went inside, they didn't smell it," Cable said. The odor wasn't strong enough to concern the adults, so they didn't report it, he said.
Shirley's boyfriend, Mark Leonard, replaced the home's thermostat about two or three weeks ago after the couple discovered the furnace wasn't working.
Investigators haven't said from which house they think the explosion originated. They said late Tuesday that they are looking at gas appliances in the homes that were consumed at the epicenter of the fiery blast, which flattened the Shirley home and the house next door where two people died.
The National Transportation Safety Board and the utility that provides gas service to the neighborhood have said they found no leaks in the gas main or pipes leading into the homes.
Real estate records show the Shirley home had a gas furnace and water heater.
But determining whether either of those caused the explosion is likely to be a long process because of the massive amount of damage.
John J. Lentini, a fire investigation expert who wrote the book "Scientific Protocol for Fire Investigation," said investigators will have to trace gas lines, valves and appliances inside homes where little more than charred boards and debris remains.
"You have to check the gas lines, and they tend to be gone," Lentini said. "You're probably not going to have much more than the meter."
In a natural gas explosion, leaking fuel builds up in an enclosed space until something ignites it, experts said. What follows is intense heat and a rush of air that creates a vacuum before the hot air rushes back in.
That rush of roiling air causes damage such as crumpled garage doors and can throw fragments of debris far away, said Siegel, an explosives expert who has taught forensic science at Michigan State University and Indiana-Purdue-University-Indianapolis.
But metal will survive the heat, Siegel said. So if a stove, furnace or water heater was the source, investigators will have some clues.
"It seldom happens that there'll be no fragment left of a gas appliance if it explodes," Siegel said. "Most often you will find large pieces of the appliance somewhere around. It may not be at the point of origin. It may be somewhere else in the house, hurtled through the wall."
Even smaller pieces, such as pieces of metal valve stems, can survive and be examined, Lentini said. Investigators can then tell what position the valves were in when the explosion occurred, and whether they might be a cause.
But sometimes no certain cause can be found.
"There are lots of explosions and fires where you have no cause, or no determined cause. And sometimes gas explosions fall into this category where you just can't figure out what caused the explosion," Siegel said.
Cable said he was skeptical that faulty appliances could have caused such a large explosion and suggested that Citizens Gas was trying to deflect blame. He said suspicion was being directed at Shirley because she was out of town when the house exploded.
"I would expect Citizens Gas to say they're not finding anything wrong whether they did or didn't at this point," he added.
Citizens Energy spokeswoman Sarah Holsapple said the utility had tested its gas lines in conjunction with the National Transportation Safety Board and the Indiana Utility Regulatory Commission.
"We feel very comfortable with our test results and both state and federal agencies have signed off on them," she said.