The 15-year-old girl who police say killed a teacher and a student and injured six others before dying by suicide at a school in Madison, Wisconsin, reportedly left behind a manifesto that investigators are reviewing .

Police identified Natalie Rupnow – who also went by the name Sophie – as the shooter late Monday. A law enforcement source told CNN that Rupnow “has grappled with issues and expressed some of them in writings that they are now reviewing.”

Rupnow was a student at the private Christian school Abundant Life. Her motive remains unclear. A manifesto was published on social media, the authenticity of which has not been confirmed by the authorities.

“A document about this shooting is currently circulating on social media, but we have not verified its authenticity,” Madison Police Chief Shon Barnes said at a news conference.

Barnes said police are still working to determine the reason for the shooting. “Right now we are still working on a motive and trying to figure out why this happened,” he noted.

Police said Rupnow’s family members cooperated in the investigation into the killings at Abundant Life, which occurred around 11 a.m. local time on Monday.

A second-grader called 911 to report the shooting, Barnes said.

“Let that sink in for a minute,” Barnes said of the young caller. Second graders are usually between seven and eight years old.

Barnes said a handgun was recovered from the scene – but police have not yet determined the weapon’s origin. “How does a 15-year-old get a gun?” he said.

Police said the shooting occurred in a classroom during a class period.

The identities of the victims were not publicly released Tuesday. Two of the six injured victims remained in critical condition, while others were in stable condition or released from the hospital.

Security cameras were installed in the school and teachers and students were trained in the lockdown measures. However, as a private school, it was not equipped with metal detectors.

The school’s director of elementary and school relations, Barbara Wiers, said at a news conference that evening that students understood that the emerging situation was not a drill and that they had “copeed with it excellently.”

“When they heard ‘lockdown, lockdown’ and nothing else, they knew it was real and they did brilliantly,” Wiers said.

Young women are far less likely to be suspected in school shootings than young men. As of Tuesday, nine suspects this year have been female, compared to 249 male suspected shooters, according to the K-12 school shooting database.

The database says it tracks all cases in which a weapon is brandished, fired or a bullet hits school grounds for any reason, regardless of the number of victims, time of day or day of the week.

The New York Times reported that the school Rupnow attended and where the shooting occurred often mentors children who have been bullied or disadvantaged at other schools.

Rebekah Smith, whose teenage daughter was in a physics class across the hall from where the shooting occurred, told the outlet that staff were trained to quickly put an end to cruelty within students.

Smith said members of the school community believe the shooter was new to the private school this year.

“You feel sorry for the parent who says, ‘Maybe this will help my child,'” Smith told the Times. “I can’t even imagine what they’re feeling.”

The police chief was questioned about online comments suggesting the shooter may be transgender.

“I don’t know if Natalie was transgender or not,” Barnes said.

“I don’t think what happened today had anything to do with how she or he or they wanted to identify,” he added. “And I wish people would leave their personal biases out of it.”