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Two suspects believed to be students used handguns and opened fire in two classrooms at the STEM School Highlands Ranch near Denver on Tuesday afternoon, authorities said. One was an 18-year-old male, the other a female juvenile, according to Douglas County Sheriff Tony Spurlock.

An 18-year-old student was killed at the school, Spurlock said Wednesday. Eight other students were shot but survived.

The FBI defines an active shooter incident as “an individual engaging in killing or attempting to kill people in a confined and populated area.”

Data from an FBI study and other reports suggest that active shooter incidents involving female shooters are rare.

What the data reveals

In an FBI list of active shooter incidents in the United States from 2000 to 2017, nine of 250 incidents identified involved female shooters.

The women in those shootings were usually armed with handguns and opened fire inside colleges, businesses or their current or former workplaces, according to the list.

But in general, there are fewer female shooters when it comes to firearm homicides, Adam Lankford, a criminal justice professor at the University of Alabama, told CNN in 2018. FBI data from 2016 showed that 7.6% of offenders who committed murder were female.

“Research shows that basically males commit more homicides than females, regardless of the subtype of homicide,” Lankford said.

There’s no one accepted definition for mass shootings. The Gun Violence Archive, which compiles data, defines it as an incident in which an offender shoots or kills four or more people. And the Congressional Research Service defines it as when the perpetrator kills four or more people, selecting victims randomly, and attacks in a public place.

But in those incidents, female mass shooters are rare.

“Men commit the overwhelming majority of mass shootings for basically the same reasons they commit most violent crimes,” Dewey G. Cornell, a licensed forensic clinical psychologist and director of the Virginia Youth Violence Project at the University of Virginia, told CNN in 2018.

“Men tend to be more violent than women because of a complex interaction of evolutionary and psycho-social factors. Men tend to be more aggressive and less inhibited by empathy, and men in distress seem to be less willing to turn to others for help,” he wrote.

None of the perpetrators behind the 28 mass attacks in 2017 was female, according to a report by the US Secret Service. A study led by Lankford, published in the journal Violence and Victims, looked at 292 public mass shooters worldwide from 1966 to 2012 and found that only one of those was female.

Why are there fewer women shooters?

There’s no single explanation for why women are rarely responsible for active shootings because there’s no single profile of a mass shooter, Cornell said.

If anything, the location of the shooting is more telling than the shooter’s gender, he said.

“There are several pathways we observe: antisocial individuals who are part of a gang, depressed and despondent persons who are highly distressed over a grievance and believe they have no future, and a small group of persons with severe mental illness that includes delusional thinking,” he said.

“In a workplace shooting, you want to look at the latter two as most likely.”

Lankford offered a similar assessment when asked why women are rarely mass shooters.

“We can’t really answer that question of differences between male and female offenders because we don’t have enough female offenders. The problem, or the good news, is we don’t have enough female offenders for a statically significant sample.”

But there have been cases in which women have carried out deadly mass shootings.

In 2018, 26-year-old Snochia Moseley shot three people and wounded three more before fatally shooting herself at an Aberdeen, Maryland, Rite Aid support facility. She was a temporary employee at the facility.

The same year, Nasim Najafi Aghdam shot and wounded three people and took her own life at YouTube’s Northern California headquarters. Police said she was upset with YouTube’s practices and policies, but she had no connection to the three victims.

A married couple, Syed Rizwan Farook, and his wife, Tashfeen Malik, massacred 14 people at a holiday party in 2015 in San Bernardino, California. Farook had worked with the San Bernardino County health department, which was hosting the party when the attack took place. They were both killed in a shootout with police.

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On January 30, 2006, Jennifer San Marco visited her former place of employment, a postal distribution center in Goleta, California, and fatally shot six employees after killing a one-time neighbor. She then killed herself.

CNN’s Christina Maxouris, Sheena Jones, Holly Yan, Emanuella Grinberg and Casey Hicks contributed to this report.