Police in Wisconsin are interviewing friends and family and examining the online presence of a 15-year-old girl to determine what drove her to open fire in a classroom at her private Christian school, killing a fellow student and a teacher before taking her own life.
The shooter, identified by police as Natalie Rupnow, who also went by Samantha, was a student at the Abundant Life Christian School in Madison, Wisconsin, the state capital.
In one key way, the Madison attack is an outlier among US school and mass shootings: Women and girls are far less likely to carry them out than male attackers. Only about three per cent of all US mass shootings are perpetrated by females, studies show.
"Identifying a motive is our top priority, but at this time it appears that the motive was a combination of factors," Madison Police Chief Shon Barnes said at a press conference on Tuesday.
Barnes did not provide more detail, saying he didn't want to jeopardise the investigation.
Investigators are examining her mobile phone, computer and online posts as well as looking into verifying a possible manifesto that the shooter may have left behind.
Some details of Rupnow's life have begun to emerge.
Police have confirmed the name of the street where she lived in Madison. Online records show someone named Jeffrey Rupnow lives on the same street, and a Facebook profile belonging to a Jeff Rupnow in Madison shows photos of a newborn daughter named Natalie Lynn from 2009, 15 years ago.
Jeff Rupnow's cover photo, posted in August, shows what appears to be a teenaged girl trap shooting with a shotgun at a local club. In a comment on the site, Jeff Rupnow says he and his child joined the club in the spring and "have been loving all every second of it".
Rupnow could not be reached for comment.
In the photo, the girl is wearing a T-shirt of the German band KMFDM. The teenaged shooters in the 1999 Columbine school massacre in Colorado were known to be avid fans of the band, and one of the killers posted the band's lyrics on his website.
At the time, KMFDM issued a statement condemning the attack, expressing sympathy for the victims and affirming its music was intended to stand against violence.
Monday's shooting took place in a study hall classroom just before 11am local time.
Previously, Barnes said the first 911 call reporting the shooting was placed by a second grade student. On Tuesday he clarified that the call actually came from a second grade teacher.
Two students remained in critical condition, while several other victims suffered less serious injuries, officials said.
Police said Rupnow used a pistol, but have not said how it was obtained. Earlier this year, the Michigan parents of a school shooter in that state were found guilty for their role in allowing their son to have access to the gun he used to kill.
Wisconsin officials said at Tuesday's press conference that it was far too early to say if Rupnow's parents could be held legally responsible. Police Chief Barnes said that for now it appeared that such charges were not warranted.
School shootings have become a near-daily occurrence in the United States, with 322 of them this year, according to the K-12 School Shooting Database. That is the second highest total of any year since 1966 - topped only by last year's total of 349.
Polling shows American voters favour stronger background checks on gun buyers, temporary limits on people in crisis and more safety requirements for gun storage at homes with children. Yet political leaders have largely declined to act, citing the US constitutional protection for gun owners.
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