CINCINNATI – Among the mysteries that investigators into the mass shooting in Dayton, Ohio, will seek to answer is whether the shooter meant to kill his sister or whether she was fatally shot trying to stop him.

Megan Betts, 22, a student at Wright State University, was among nine people who died early Sunday in the Oregon District of bars and restaurants near Dayton’s downtown.

The murder of a sister is called a sororicide. It’s not a well-studied phenomenon in the United States.

A 2013 FBI study of 160 mass shootings between 2000 and 2013 found that in nine cases, shooters killed relatives – parents, spouses, children – before going into a public place to continue firing. A 1981 study of “honor killings” in the Middle East concluded that in about one-third of instances, brothers killed sisters.

Did Dayton gunman target his sister?The Idea 'defies believability,' police chief says

The Cincinnati Enquirer asked two experts Monday about the family connection in Sunday’s mass murder. They said it is too early to see the factors that led to Megan Betts’ death apparently at the hands of her brother, Connor Betts, 24, of Bellbrook, Ohio.

“Boy, that’s a tough one,” said Randolph Roth, an Ohio State University historian whose specialty is murder. “We don’t know, forensically, the sequence of events. We don’t know if she was shot trying to stop him.”

Dayton police have said Megan Betts and a male friend spent a portion of Saturday night with her brother, and the trio traveled together to the Oregon District. At some point, Connor Betts separated from his sister, but when he returned to the scene, he was armed and firing. Police responded within a minute of the first shots.

What we know about the gunman:Dayton shooter who police say killed 9 was in a 'pornogrind' metal band

Police said Megan Betts was among the first victims. The male friend, shot in the torso, was among 14 people injured in the shooting. Dayton Police Chief Richard Biehl said Monday there was no indication Megan Betts or the male friend knew of the weapons. He said Connor Betts’ intentions toward his sister during the shooting are not known.

“It just seems to defy believability that he would shoot his own sister, but it is also hard to believe he didn’t recognize his sister,” Biehl said.

OSU’s Roth said it is likely Megan Betts did not know of her brother’s weapons or his plan. “I’m sure the last thing on his sister’s mind was that he was going to do this. If the people closest to you can’t tell, what are we supposed to do? We’ve got to limit the firepower.”

Dr. Eileen Ryan is a forensic psychiatrist and vice chair of clinical services in the department of psychiatry and behavioral health at OSU. She pointed out Monday there are few studies on mass shootings, much less the killing of a sister in a mass shooting.

“It’s sort of human nature to cast about for a reason,” Ryan said. “In this case, you look at the fact that his sister was among the dead, it raises the issue of the sister, and perhaps (Connor Betts) not liking her boyfriend, which a very simplistic kind of view of what went on. There are plenty of brothers who don’t like their sisters’ boyfriends who don’t engage in this type of behavior.”

Connor Betts:Who is the 24-year-old man police say killed 9 – including his own sister – in Dayton, Ohio?

In the hours after the Dayton shooting and the Saturday shooting in El Paso, Texas, that killed 22 people, President Donald Trump and others have attributed deaths to mental illness. But Ryan said: “We know that in the vast majority of these mass murders, they are not committed by people who have mental illness. In terms of mental illness, the risk of someone mentally ill for violence is not much higher than the rest of the population.”

Ryan also noted the “contagion” of mass shootings. The FBI’s 2013 study estimated that the number of shooting was rising 16% a year. The Dayton event was the 251st in the United States in 2019. “These mass murders tend to be committed by very alienated and – angry is too soft a word – enraged young men.”

“Violence is like fever,” she said. “It’s a very nonspecific sign or symptom, and it could be relative to a whole variety of things, including things that aren’t related to mental illness. Prisons are full of people who are violent but not mentally ill.”

Follow Anne Saker on Twitter: @apsaker