Public Safety Director Troy Riggs said investigators will treat the area as a crime scene until they rule out foul play.
The blast forced about 200 people out of their homes in the once-tidy neighborhood of one- and two-story single-family houses. Some have been allowed to reoccupy their homes, and others have been escorted in to retrieve valuables and other belongings. Adam Collins, the city's deputy code enforcement director, said 29 remained uninhabitable Monday.
Mark Karnes, whose house is four doors down from the blast site and suffered severe structural damage, hoped to retrieve clothes and look for his cat. But he also questioned the wisdom of going back inside the house given the extent of the damage.
"Because the walls bowed out and separated from the ceiling, I don't think it's safe," he said.
The blast flattened the house Shirley co-owns with his ex-wife and one next door that belonged to second-grade teacher Jennifer Longworth and her husband, John. Indianapolis police said Monday the bodies of the pair were found in the basement of their home, which was leveled in the blast.
A candlelight vigil was held Sunday night at the school where Jennifer Longworth teaches. Her husband's employer, consumer electronics company Indy Audio Labs, issued a statement Monday saying it was "saddened by the loss."
Greenwood Community Schools Superintendent David Edds said Jennifer Longworth had taught at Southwest Elementary School for 12 years. Her husband had worked at Indy Audio Labs for 10 years and was director of product development and technology, according to the company.
John Shirley said Jennifer Longworth was quiet but funny and her husband was a huge Indianapolis Colts fan who maintained a garden of beautiful wildflowers along the side of the house.
"They were just very sweet people," he said.
Indiana real estate records show Shirley's house had been for sale for a year until it was taken off the market in March.
Associated Press researchers Lynn Dombek and Rhonda Shafner in New York contributed to this report.