Story highlights Two brothers among accused in explosion at that killed couple next door

Explosion injured 12 other people and caused $4 million in damage

Investigators found abnormalities in gas line at suspect's home, affidavit says

Three Indiana residents were arrested Friday on charges of arson and felony murder in a fatal house explosion in Indianapolis in November, according to the Marion County, Indiana, prosecutor's office.

Monserrate Shirley and brothers Mark Leonard and Bob Leonard Jr. were arrested Friday morning and charged with two counts of felony murder in the deaths of John and Jennifer Longworth, the neighbors of Shirley and Mark Leonard who were killed by the explosion at Shirley's home in the Richmond Hill area on November 10.

"We have to acknowledge that we are helpless to alleviate the pain and anguish of such innocent victims and their families," prosecutor Terry Curry said at a news conference Friday, according to a news release. "However, what we as a public safety community can do and must do is devote our best efforts to see that justice is served on behalf of those victims."

The explosion also injured 12 people and caused roughly $4 million in residential damage, according to the prosecutor's office.

An investigation led by local and federal authorities, including the Bureau of Alcohol, Tobacco, Firearms and Explosives, revealed suspicious behavior and circumstances surrounding the explosion. In particular, various elements of the gas line at Shirley's home may have been tampered with before the blast, including a missing step-down regulator, which reduces gas pressure entering the home, and a missing on/off valve to the residence's gas fireplace.

"The absence of the valve and regulator would explain how such a large volume of gas would be released into the home," Curry told CNN.

In addition, the digital thermostat at Shirley's home had been replaced by a slide-switch thermostat that "will produce a spark when the thermostat reaches a specified temperature," the probable cause affidavit says. However, it appears that the thermostat did not contribute to the explosion; instead, it appears to have been started by the microwave in the kitchen, which was a model that could be programmed to start 24 hours in advance.

Witnesses interviewed during the investigation indicated that the three accused may have a long history of fraud, particularly Mark Leonard, who has allegedly been participating in confidence schemes since 1995, according to the affidavit. The investigation also revealed that Shirley filed for bankruptcy in 2012.

In addition to two counts of felony murder, the accused are charged with conspiracy to commit arson, 12 counts of arson as a Class A felony and 33 counts of arson as a Class B felony. Mark Leonard and Shirley were also charged with conspiracy to commit arson as a Class B felony. They are being held at the Marion County Jail without bail, and an initial court hearing is scheduled for Monday morning.