By Vic Ryckaert, Indianapolis Star
INDIANAPOLIS - Pieces of insulation, siding and drywall were scattered several blocks away after a deadly explosion destroyed homes in the Richmond Hill neighborhood late Saturday.
To most people, that debris looks like random hodge-podge of junk. To a trained eye, that junk trail provides the first clue to figuring out what happened.
"They will map out all the debris, see which direction it is going," said Frank Hsu, a California-based expert in explosion reconstruction. "They mark it with arrows to show the direction of travel. That will show where the explosion came from."
PHOTOS: Indianapolis neighborhood explosion
Several experts say piecing together what happened last weekend will be a deliberate and painstakingly slow process. They are looking for debris, strange smells, suspicious people. Anything out of the ordinary. Anything that might be a clue.
They likely are using high-tech equipment as well as old-fashioned detective work - they are interviewing everyone who lives in the neighborhood or was visiting that night.
Early signs are pointing to a natural gas explosion but the investigation into the blast that killed two and injured seven is far from complete, Marion County Public Safety Director Troy Riggs said.
But what happened? Was it an accident? Was it deliberate? Experts on the ground are working to find those answers.
"It's just kind of like peeling an onion," said Donn A. Altmann, president of the Indiana chapter of the National Association of Professional Insurance Investigators. "You have to start peeling back the layers until you find out what caused the thing."
State and local arson investigators, Bureau of Alcohol, Tobacco, Firearms and Explosives agents, gas company representatives and insurance investigators remained in the neighborhood on Wednesday. They've declined to be interviewed.
Insurance investigators will be looking for evidence of faulty appliances or products that might share the blame, Altmann said. They are also looking for any possible financial motives that might lead someone to want to blow up their home.
Who might have had credit problems? Was the debt on the mortgage higher than the home was worth? Was there a foreclosure?
"I'm sure everyone is looking at those things real hard," Altmann said. "Those would be fraud indicators, but that doesn't mean there is fraud."
Folks at the scene and directly involved in the investigation are not talking. Riggs said they want to preserve evidence in case this investigation ends up in a criminal court.
"Everybody's very concerned," Riggs said. "If it is criminal, we want to make sure we are doing it the right way."
But outside experts say while gas is the leading suspected cause, the blast was not typical of most gas explosions.
"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 in Indianapolis. "I've never heard of one causing this big a blast."
On July 21, 1997, a gas explosion flattened six homes and damaged 80 more in the Charter Pointe neighborhood on the city's Northeastside. Gladys Mills, 86, was killed by the explosion. Investigators determined the blast was caused by a 20-inch gas pipeline ruptured by a contractor working for the gas company; Citizens Gas and Coke Utility paid out $1.15 million to more than a dozen victims.
More than a dozen home explosions linked to natural gas have occurred in the past two years.
They usually involved a single home, though more devastating blasts tied to pipelines have been reported - 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.
A gas leak in a Colorado home last month sparked an explosion that sent five people to a hospital and damaged several homes.
John Erickson, vice president of the American Public Gas Association, 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 ignitions 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 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.
And Hsu, the expert from California, said an explosion this "violent" would have required so much fuel that it is likely someone would have smelled something before it blew up.
"It mixed very well before it ignited. When there are people in the house, how could that not be noticed?" Hsu said. "There might be an easy explanation, but we don't know."
There's a criminal investigation going on, as well. Police are talking to people and looking for anything out of the ordinary.
"It's painstaking and that's probably what's frustrating to the community and the victims," said Mike Crooke, a retired Indianapolis police homicide detective who is now Cumberland's police chief.
"The average person looks at something and it is devastation," Crooke said, "but for those trained and specialized in this area, they can see clues that point them in one direction or eliminate another."
Investigators often use high-tech equipment and computers to help solve the puzzle, he said.
"They can rebuild these things like you wouldn't believe," Crooke said. "There are computer programs that are just fantastic."
Investigators will be thorough, too.
"They'll be talking to neighbors. They are asking questions not only directed at that night but prior nights," Crooke said.
Maybe neighbors noticed funny smells, out-of-place vehicles, or someone who just didn't belong.
Detectives will be suspicious of everyone. They'll gather background information on the dead, the injured, the homeowners, the visitors.
"It would be nice if it turns out this is an obvious malfunction here that takes the criminal element out," Crooke said. "But with that much devastation out there, wow. It's awful."
The Associated Press contributed to this report.