MADISON, Wis. (AP) — The motive for a shooting that killed a teacher and a student and wounded others at a Wisconsin school appears to be a "combination of factors," a police chief said Tuesday as he appealed to the public to share what they might know about the 15-year-old girl who attacked a study hall before shooting herself.

Madison Police Chief Shon Barnes offered no details about a possible motive, though he said bullying at Abundant Life Christian School would be investigated.

Barnes said police are also investigating writings that may have been penned by Natalie Rupnow and could shed light on her actions.

"Identifying a motive is our top priority, but at this time it appears that the motive is a combination of factors," the chief told reporters.

Barnes gave the number to a tip line for anyone who might have known the shooter and her feelings.

"There are always signs of a school shooting before it occurred. We're looking into her online activity," he said.

In addition to the deaths, six people were wounded, including two students who remain in critical condition. The shooter died of a self-inflicted gunshot.

Barnes made remarks at a news conference but left without taking questions from reporters, leaving the Madison mayor and Dane County executive to face the media. They declined to disclose the names of the victims.

"Leave them alone," Mayor Satya Rhodes-Conway snapped.

Abundant Life is a nondenominational Christian school — prekindergarten through high school — with approximately 420 students in Madison, the state capital.

Mackenzie Truitt, 24, placed a red poinsettia plant at the school to honor the victims Tuesday. She said her brother is a graduate and some of his friends were wounded.

"My heart sunk because I know how awesome a lot of these kids are," Truitt said. "I know how scared everybody was. Couldn't get a hold of certain people. Just really scary having to deal with that."

Barbara Wiers, the school's director of elementary and school relations, said when they practice safety routines, leaders always announce that it's a drill. That didn't happen Monday, the last week before Christmas break.

"When they heard, 'lockdown, lockdown,' they knew it was real," she said.

Wiers said the school does not have metal detectors but uses cameras and other security measures.

Barnes said police were talking with the shooter's father and other family members, who were cooperating, and searching the shooter's home.

The shooter's parents, who are divorced, jointly shared custody of their child, but the shooter primarily lived with her 42-year-old father, according to court documents.

Barnes said the first 911 call to report an active shooter came in shortly before 11 a.m. from a second grade teacher — not a second grade student as he reported publicly Monday.

First responders who were in training just 3 miles (about 5 kilometers) away dashed to the school for an actual emergency, Barnes said. They arrived three minutes after the initial call.

Investigators believe the shooter used a 9mm pistol, a law enforcement official told The Associated Press. The official spoke on the condition of anonymity because they were not authorized to discuss the ongoing investigation.

Carolyn Griese, 70, of Monona, didn't have any affiliation with the school, but she felt moved to drive over Tuesday. She cried as she placed flowers on the sidewalk.

"When I was growing up, we worried about the atomic bomb," she said. "And now they actually practice active shooter scenarios. And I think how could that be? They're children. Innocent children."

Griese's daughter is an associate principal in a different school. The shooting pulled her thoughts to her own grandkids, and the thought of losing family so close to Christmas.

"It's terrifying," she said. "It's horrible."

Vice President Kamala Harris addressed the shooting during a speech to students in Maryland on Tuesday.

"Our nation mourns for those who were killed and we pray for the recovery of those who were injured," she said.

Harris also called for stronger gun control laws.

Madison resident Cristian Cuahutepitzi, 32, said he attended a vigil Tuesday evening to let the families of the victims know "we're thinking of them." He said his uncle's two daughters go to the school.

"They're still a little bit shook," he said.

Joe Gothard, the superintendent of the Madison Metropolitan School District, said at the vigil that the tragedy happened less than two blocks away from his childhood home. He said it wasn't enough to say the district would work on safety.

"We need to connect like we are tonight, each and every day and make a commitment that we know we're there for one another, hopefully to avoid preventable tragedies like yesterday," he said.

The school shooting was the latest among dozens across the U.S. in recent years, including especially deadly ones in Newtown, Connecticut; Parkland, Florida; and Uvalde, Texas.

The shootings have set off fervent debates about gun control and frayed the nerves of parents whose children are growing up accustomed to doing active shooter drills in their classrooms. But school shootings have done little to move the needle on national gun laws.

School shootings by teenage females have been extremely rare in U.S. history, with males in their teens and 20s carrying out the majority of them, said David Riedman, founder of the K-12 School Shooting Database.

In a blog post last year, he mentioned a handful of school shootings that involved perpetrators who identified as female, including a 16-year-old girl who shot and killed two and injured several others at San Diego elementary school in 1979.

Firearms were the leading cause of death among children in 2020 and 2021, according to KFF, a nonprofit that researches health care issues.

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Associated Press writers Alanna Durkin Richer, Ed White, Josh Funk, Hallie Golden and Ryan Foley and photographer Morry Gash contributed to this report.