The turn of events came as a funeral was held for the two people killed in the Nov. 10 blast, Dion and Jennifer Longworth, both in their mid-30s, The Indianapolis Star reported. They lived next door to the home of Monserrate Shirley, 47, where the explosion occurred.
Marion County Prosecutor Terry Curry called on the public to call in with any tips about the blast, which caused more than $4 million in damage, the newspaper said. He said investigators wanted to learn more about a white van seen in the vicinity before the explosion.
"At this time we're here to inform you that we're turning this into a criminal homicide investigation," Gary Coons, chief of the Indianapolis Department of Public Safety's Homeland Security division, told reporters.
No further details of the criminal investigation were provided.
Authorities had yet to determine exactly how the explosion was set off, though natural gas appeared to have been involved.
The Star said Shirley and her boyfriend, Mark Leonard, were out of town, her daughter was with friends and their cat had been boarded when the blast occurred. Neighbors said a truck that had been parked in front of the house for some time was moved just hours before the explosion.
Shirley has said she had no idea how the explosion occurred. Her ex-husband, John Shirley, mentioned there had been a problem with the home's furnace, but she said a thermostat was replaced and there had been no further issues with the heating system, the Star said.
The Shirleys had filed jointly for Chapter 13 bankruptcy, showing liabilities of nearly $410,000 and assets of about $316,000. They divorced in February 2011.
The house, valued at $230,000 with a mortgage of about $225,000, was put up for sale for $149,900 later that year but was pulled off the market in March, the Star said.
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