(CNN) — Police are investigating why a teenage girl brought a handgun to her small Christian school in Wisconsin and opened fire, killing two people and wounding several others a week before Christmas.

A student and a teacher were killed, and six people were taken to area hospitals, Madison police said. The slain victims haven’t been publicly identified.

Madison police identified the shooter as 15-year-old Natalie Rupnow, a student at Abundant Life Christian School. Evidence suggests the shooter, who also went by the name “Samantha,” died from a self-inflicted gunshot wound, Police Chief Shon Barnes said.

LIVE COVERAGE: Authorities identify suspect in Madison, Wisconsin, school shooting

Here’s what we know about the shooting and the investigation:

How the deadly attack unfolded

Hours before the shooting began Monday, students from kindergarten to high school filed onto the school’s 28-acre campus. They had a week of festivities to look forward to, including a holiday concert and an Ugly Christmas Sweater Day, according to the school’s website.

The shooter entered the school at the beginning of the day with the rest of the students, police said.

Late Monday morning, the shooter was in a study hall classroom with students from mixed grades, the police chief said. The teen pulled out a handgun and opened fire, unleashing bullets that were “confined to one space,” Barnes said.

At 10:57 a.m., a second-grade teacher called 911 to report a school shooting, Barnes said. Previously, Barnes had said a second-grade student had called 911 before correcting himself Tuesday.

A deputy from the Dane County Sheriff’s Office was the first to arrive on the scene at 11 a.m. The first Madison police officer arrived 24 seconds later and immediately entered the school.

A group of police medics were in a training session on mass trauma incidents about 3 miles away, Barnes said.

“They left the training center immediately and came down here — and doing in real time what they were actually practicing for,” Barnes said.

“When officers arrived, they found multiple victims suffering from gunshot wounds.

How students responded

The police medics weren’t the only ones trained to respond to a mass shooting. The children at Abundant Life had previously practiced shooter drills before Monday’s tragedy.

“When they heard ‘lockdown, lockdown’ and nothing else – they knew it was real,” said Barbara Wiers, director of elementary and school relations for Abundant Life.

“They were clearly scared,” Wiers said, “but they handled themselves brilliantly.”

Sixth grader Adler Jean-Charle was in class with his twin brother at the time of the shooting. He recalled hearing two gunshots and asking the same question many across America are wondering now: “Why did they do that?”

“We heard them and then some people started crying,” Adler said. “Then we just waited until the police came to escort us out to the church.”

The attack at Abundant Life is at least the 83rd school shooting of 2024, surpassing 2023 for the most school shootings in a single year since CNN began tracking such shootings in 2008.

Of the 83 school shootings this year, 56 have been reported on K-12 campuses and 27 on university and college campuses. The deadliest school shooting of the year was in Winder, Georgia, where four victims were left dead at Apalachee High School.

“I hoped that this day would never come in Madison,” Mayor Satya Rhodes-Conway said Monday. “It is not something that any mayor, any fire chief, any police chief, any person in public office ever wants to have to deal with.”

What we know about the shooter and the investigation

Natalie Rupnow attended the private school along with some 420 students in grades kindergarten through 12, officials said. The school’s website boasts “smaller class sizes” catering to students from about 200 families in the Dane County area.

Federal and local investigators are trying to determine the shooter’s motive and where the teen’s gun came from.

Investigators haven’t revealed any prior encounters with Rupnow or any known problems. Police have been speaking with Rupnow’s family members, who are cooperating with the investigation.

Police are looking into whether the teen shooter’s parents owned the gun, Barnes said Tuesday.

“We also want to look at if the parents may have been negligent. And that’s a question that we’ll have to answer with our district attorney’s office,” Barnes said. “But at this time, that does not appear to be the case.”

Madison police are also working with the Bureau of Alcohol, Tobacco, Firearms and Explosives to find the origin of the gun.

“We have asked our partners with the ATF to expedite what’s called an ATF trace form to try and determine the origin of that weapon, who purchased it and how it got from a manufacturer all the way to the hands of a 15-year-old girl,” Barnes said. “These are questions that are going to take some time to answer.”

When asked about the shooter’s gender identity, police emphasized the need for greater responsibility on social media regarding posts about the shooter, and urged people on the internet to leave out their personal biases.

“I don’t know whether Natalie was transgender or not. And quite frankly, I don’t think that’s important at all,” Barnes said.

Investigators are looking into online posts and a possible manifesto that might be linked to the shooter, the police chief said Tuesday.

“We have been made aware of a manifesto, if you want to call it that, or some type of letter that’s been posted by someone who alleged to be her friend. We haven’t been able to locate that person yet, but that’s something we’re going to work on today,” Barnes said.

“We’ll also be looking through her effects – if she had a computer or cell phone – to see if there are any transmissions between her and someone else.”

Law enforcement officials are working to interview witnesses, and students are being encouraged to speak only when they feel ready, the police chief said.

“We’re not going to interrogate students. We’re going to give them an opportunity to come in and speak to what they may have saw when they feel ready,” Barnes said.

Local and national leaders call for action

President Joe Biden called for more action to address gun violence after yet another tragic school shooting.

“From Newtown to Uvalde, Parkland to Madison, to so many other shootings that don’t receive attention – it is unacceptable that we are unable to protect our children from this scourge of gun violence,” Biden said in a statement Monday. “Every child deserves to feel safe in their classroom.”

He called on Congress to “pass commonsense gun safety laws,” including universal background checks, a national red flag law and a ban on assault weapons and high-capacity magazines.

“We can never accept senseless violence that traumatizes children, their families, and tears entire communities apart,” Biden said.

Barnes, who was a teacher before he became a police officer, said he was devastated to learn students had been shot in his own city. He called for greater action to prevent such tragedies, often seen on the news, from becoming reality in other communities across the country.

“We have to come together to do everything we can to support our students, to prevent press conferences like these from happening again and again and again,” Barnes said. “I think we can all agree that enough is enough.”

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