Story highlights Women make up only about 5% of mass killers, a criminologist says

Police say Tonya Thomas shot her four children, then killed herself early Tuesday

911 calls recount confusion over the shootings in Port St. John, Florida

Neighbors have described a history of public arguments among Thomas and her children

The early-morning slaughter of four Florida siblings at the hands of what authorities say was their mother is an "almost unheard of" case, crime experts said Wednesday.

Investigators are still trying to determine what happened at the Port St. John home of 33-year-old Tonya Thomas early Tuesday morning. But Patricia Pearson, author of "When She was Bad: How and Why Women Get Away with Murder," said mass killings with women as a perpetrator are rare -- and when they do happen, they typically aren't committed with guns.

"If a gun was used, that's almost unheard of," Pearson told CNN.

Women make up about 5% of the ranks of mass killers, said James Alan Fox, a criminologist at Northeastern University. And most cases in which women kill their families involve young children, said Fox, the author of "Extreme Killing: Understanding Serial and Mass Murder."

Sheriff's deputies in Brevard County say Thomas turned a .38-caliber revolver on herself after killing her two sons, 15-year-old Jaxs Johnson and 12-year-old Joel Johnson, and two daughters, 17-year-old Pebbles Johnson and 13-year-old Jazlin Johnson. The children went to a neighbor's house when the shooting started, with at least one reporting being shot, according to recordings of 911 calls released by the sheriff's department Tuesday -- but they returned while neighbors were on the phone with deputies, and more shots followed.

"The two kids were in the front yard, one banging on the front door, 'Help us! Help us!'" a neighbor told dispatchers. "There was so much confusion going on we couldn't tell who shot who, and then when I was talking to you the one kid said his mom shot him."

Investigators don't know a motive for the killings, said Lt. Tod Goodyear, a spokesman for the Brevard County Sheriff's Office. But he said neighbors have described a history of public arguments among Thomas and her children, and Jaxs Johnson was arrested after a pair of fights with his mother in April.

The teen was charged with domestic violence after a pair of fights with his mother, according to records released by the sheriff's office. In the first, on April 8, he knocked out a window with his bicycle during a fight; Thomas told deputies "she is attempting to get him into different programs to help with his issues."

The following day, she said, her son punched and kicked her when she tried to wake him for school, then ran out of the house. She again called deputies, who arrested him on April 10, Goodyear said. He spent at least two days in juvenile detention after the arrest and had a court date scheduled for Tuesday, Goodyear said.

Thomas herself had been arrested on domestic violence charges in 2002, after a fight with her former boyfriend, the children's father, Goodyear said.

Fox said the circumstances appear to be a case of "suicide by proxy," in which a family member takes the lives of her loved ones "out of a warped sense of love" before killing herself.

"Typically, the perpetrator is suicidal, feels life is miserable and doesn't want to go on," Fox said. "But why does she take her children? Because she wished to be reunited them in the afterlife or wants to spare them the misery of this life."

Usually the victims in such cases are young children, however -- "partly because young children are vulnerable," he said. "Older kids can run away."

And the reported circumstances are "really unusual," far different from typical cases involving either women or men.

In an e-mail to CNN, Pearson said that in cases where women kill their children, "They target them because that is their sphere of power, and also their source of stress." But she added, "I've never seen a case where the children were this old, however. They're almost always under 5."