The incident was originally thought to have been caused by a faulty furnace, one of the homeowners said.
Indianapolis Homeland Security Director Gary Coons announced the criminal investigation Monday evening, shortly after a funeral was held for the husband and wife (pictured below) who had lived next door to the house where investigators believe the blast occurred.
"We are turning this into a criminal homicide investigation," Coons said after meeting with residents, the first public acknowledgement by investigators of a possible criminal element to the Nov. 10 explosion.
Search warrants have been executed and officials are now looking for a white van that was seen in the subdivision on the day of the blast, Marion County Prosecutor Terry Curry said. Federal authorities are offering a $10,000 reward for information in the case.
Curry said the investigation is aimed at "determining if there are individuals who may be responsible for this explosion and fire," but neither he nor Coons took questions or indicated if investigators had any suspects. No arrests have been made.
A lawyer representing Monserrate Shirley and Mark Leonard, who lived in the home that is believed to have exploded, said Tuesday that the couple was bewildered by the new direction of the investigation.
Shirley said in a video on the Indianapolis Star newspaper's website last week that investigators had asked her whether she had any enemies who wanted to kill her, suggesting that someone possibly planted a bomb in her home.
Shirley cried in the video and said that people were blaming her for the blast.
"Everybody's pointing a finger at me like I did something wrong," Shirley said. "I mean, I'm totally devastated like my neighbors are.
"It's like waking up to this bad dream," she continued. "I mean, sometimes I was there and I'd be dead, and I wouldn't have to be asked so many questions."
Randall Cable said in a statement that Shirley and Leonard have "cooperated fully" with investigators and that they want the cause "of this horrific and saddening tragedy to be determined."
Officials say they believe natural gas was involved in the explosion, which destroyed five homes and left dozens damaged. Investigators have focused on appliances in their search for a cause. The explosion caused an estimated $4.4 million in damage.
"We thought something like this was not just an accident," said Doug Aldridge, who heads the neighborhood Crime Watch.
Aldridge said he and other residents frequently saw a white van parked outside the home, though he didn't know who owned it. He said residents are angry and upset but that he expects most of them to stay in the neighborhood.
Hundreds of people attended the funeral Monday for John Dion Longworth, 34, and his 36-year-old wife Jennifer Longworth.
She was a second-grade teacher remembered for knitting gifts for her students, while her husband, an electronics expert, was known as a gardener and nature lover. The school where Jennifer Longworth taught was closed Monday so teachers and students could attend the funeral.
Indianapolis Mayor Greg Ballard told reporters after attending the Longworths' funeral Monday that he had been having a hard time coming to terms with what happened.
"There is a search for truth and there is a search for justice," Ballard said.
John Shirley, who co-owns the house with his ex-wife, Monserrate Shirley, has told The Associated Press that he had recently received a text message from his 12-year-old daughter saying the furnace in the home had gone out.
Monserrate Shirley said that Leonard had replaced the thermostat and that the furnace was working. Cable has said the daughter told her mother she had smelled an odd odor in recent weeks, but they hadn't reported it.
Shirley and Leonard were away at a casino at the time of the blast, Cable said. The daughter was staying with a friend, and the family's cat was being boarded.
AOL Real Estate contributed to this report.
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