Mark Leonard, Monserrate Shirley, and Robert Leonard were charged today with murder and arson in the Nov. 10 home explosion that killed two people. (Photo: Indianapolis Metropolitan Police)

Prosecutors in Marion County, Indiana today announced murder and arson charges against three people in connection with a deadly home explosion Nov. 10 that set off a chain reaction of fires, leaving a neighborhood in ruins - and a young couple dead.

Monserrate Shirley, 47, who owned the home that exploded, was charged along with her live-in boyfriend, Mark Leonard, 43, and his brother, Robert Leonard, 54.

The defendants will have their initial hearings on Monday morning in Marion County Superior Criminal Court.

Authorities allege that the November blast in the Richmond Hill neighborhood of Indianapolis was an intentional act, a conspiracy concocted by the three to collect on a recently-increased $300,000 insurance policy that covered the personal property in the home.

Marion County Prosecutor Terry Curry called it a "thoroughly, thoroughly, senseless act."

In affidavits unsealed today, investigators allege that the trio released natural gas into the home for six to nine hours. The source of ignition for the blast was a microwave oven - allegedly set on a timer to ignite the gas. Additionally, a regulator restricting gas flow into the home appeared to have been removed, and a valve that controls the flow of gas to a fireplace is missing, authorities said.

Shirley and Mark Leonard were away from the home on the night of the blast, staying overnight at a nearby casino. Shirley's 12-year-old daughter had been sent to stay with a babysitter, and the family cat spent the weekend with boarders.

Investigators allege that there was a nearly identical, but failed, attempt to blow up the house a week earlier. The cat was boarded, the daughter sent to the same babysitter and Leonard and Shirley took off for the same casino.

Mark Leonard, prosecutors allege, had called a friend that weekend to say, "The house blew up." Leonard then told the friend that he was surfing the Internet "looking for a Ferrari to buy," according to an affidavit unsealed today. Leonard allegedly told the friend that Shirley "has jewelry insured and they expect to get $300,000 and he would get $100,000 of it." It was only later, prosecutors say, that Leonard discovered that the house had failed to explode that time.

Curry said today that as soon as investigators began poring over the remnants of Shirley's home, they noticed that the microwave oven had been blasted out from the inside. A small canister found in the kitchen may have been inside the microwave, but authorities say is still being tested for evidence of flammable contents.

When the home exploded on Nov. 10, the noise of the explosion could be heard and felt for miles around - and it sent a blast wave into the home next door, killing John and Jennifer Longworth. A coroner's report found that John Longworth, 34, died from burns covering more than 95 percent of his body. Jennifer Longworth, a 36-year old schoolteacher, died from blast injuries to the head and burns that covered 80 percent of her body. She was found dead in her bed, which had collapsed through the floor to the basement.

Curry said authorities are still seeking at least one other person who, they believe, may have had a role in the crime. That person was allegedly seen entering the home on the day of the explosion, along with a man matching the description of Robert Leonard.

In addition to two counts of felony murder, all three defendants are charged with conspiracy to commit arson, and 45 counts of arson for causing injuries to others and damage to neighboring homes. The total damage has been estimated to be in the range of $4 million.

The case qualifies for the death penalty, but Curry said no decision has yet been made on whether to seek it.

Randall Cable, an attorney for defendant Monserrate Shirley, did not immediately respond to emails and phone messages.