Instead of being crushed, the microwave oven had blown apart from the inside out, investigators reported. A timer, they theorized, could have been set to turn the microwave on hours after everyone had left, sparking the gas explosion.

After shoveling through the debris, investigators also said they were unable to find a crucial gas valve and another regulator part that would have limited the flow of gas from Ms. Shirley’s fireplace, suggesting both may have been removed. Testing of gas mains throughout the neighborhood indicated no sign of accidental leaks.

Within two weeks, officials believed they had enough evidence to announce that a homicide investigation was under way. By then, public suspicion had risen about the involvement of Ms. Shirley and Mr. Leonard.

“It’s like waking up to this bad dream,” Ms. Shirley said, crying as she spoke to The Indianapolis Star in November. “I mean, sometimes I wish I was there and I’d be dead and now I wouldn’t have to be asked so many questions.”

An examination of Ms. Shirley’s background revealed a history of troubled finances and, for the police, a possible motive. In 2007, she and her husband filed for bankruptcy. Divorce documents showed that Ms. Shirley took control of two mortgages — now totaling more than $200,000 — and the bankruptcy payments after the couple split last year. She currently owes an additional $63,000 in credit card debt, investigators learned.

Randall L. Cable, a lawyer for Ms. Shirley, did not respond to requests for comment, but in an interview before Ms. Shirley and Mr. Leonard were arrested on Dec. 21 he said that they were “as bewildered as everyone else” about the explosion. “It’s always neater if they can blame somebody,” Mr. Cable said, about police and news reporters, denying his client’s involvement.

Court documents show that circumstantial evidence against the couple continued to mount, including a $304,000 homeowners’ policy on Ms. Shirley’s personal property; an account of a conversation between Mr. Leonard and a friend suggesting a failed attempt to detonate the house a week before the explosion; and evidence that a large plastic tote containing Ms. Shirley’s family photographs and other personal items was given to Mr. Leonard’s nephew for storage.