INDIANAPOLIS - A homeowner, her boyfriend, and the boyfriend's brother were charged Friday with murder and arson in a massive explosion that killed two people, destroyed five homes, and damaged dozens more in an Indianapolis neighborhood.

Monserrate Shirley, Mark Leonard, and Bob Leonard were arrested Friday and charged with felony murder in the Nov. 10 blast, Marion County Prosecutor Terry Curry said at a news conference. They also face felony arson and conspiracy charges and have been jailed without bond, Curry said.

Investigators determined that Shirley's home - at the epicenter of the blast that killed her neighbors John and Jennifer Longworth - filled up with gas after a gas fireplace valve and a gas line regulator were removed, Curry said. A microwave, probably set to start on a timer, sparked the explosion and flattened much of the Richmond Hills subdivision, he said.

Curry called the explosion a "thoroughly senseless act."

Randall Cable, the attorney for Shirley and Mark Leonard, said he was stunned. "I'm just as surprised as everyone else that they've made an arrest. My clients have consistently indicated their innocence," he said.

Investigators found Shirley and Mark Leonard had tried to blow up the home a week earlier, and that Leonard had told an acquaintance the house and Shirley's jewelry were insured for $300,000, Curry said.

A man fitting Bob Leonard's description was seen at Shirley's home on the day of the explosion, and investigators believe this is when the gas line and valve were tampered with.

Cable has said the couple were at a southern Indiana casino when the explosion happened. Shirley's daughter was staying with a friend, and the family's cat was being boarded.

Curry said Bob and Mark Leonard told investigators they had last seen each other four days before the explosion, but investigators found surveillance video from two businesses showing them together on the two days before the blast.

The day before the blast, Curry said, the brothers allegedly spoke with an employee of local gas utility Citizens Energy.

They "asked that person various questions regarding gas, including the differences between propane and natural gas, the role of a regulator in a house and controlling the flow of natural gas and how much gas it would require to fill a house," he said.

The blast destroyed five houses including the Longworths' home, and damaged about 90 more. John Longworth was an electronics expert and Jennifer Longworth was a schoolteacher.