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(NewsNation) — Authorities identified the shooter who opened fire during morning Mass at Annunciation Catholic School in Minneapolis as 23-year-old Robin Westman, who legally purchased multiple firearms and left behind a “manifesto.”

FBI director Kash Patel said the organization is investigating the shooting “as an act of domestic terrorism and hate crime targeting Catholics.”

The shooting took place during an all-school Mass at the Annunciation School, a private school on the south side of the city.

Minneapolis Police Chief Brian O’Hara said he couldn’t confirm if the attack was directed against Catholics. Officials said Westman acted alone, and a motive is still under investigation.

Mayor Jacob Frey urged against using the tragedy to target the transgender community.

Minneapolis school shooter wrote anti-Trump, religious messages on rifle

The Department of Homeland Security confirmed that certain phrases, including “Kill Donald Trump,” were written on one of Westman’s rifle magazines.

Investigators said the writings matched portions of a video “manifesto” posted online before the attack, which has since been removed by the FBI.

O’Hara confirmed there was “some sort of manifesto that was timed to come out on YouTube. It’s been taken down, and our investigators are going through that to try to develop a motive.”

In the video, Westman can be seen filming several guns, including a rifle and a pistol, and a bevy of bullets and gun magazines for what appears to be a detailed, planned attack.

The firearms and magazines had notes and drawings scrawled in a silver pen. 

One read “kill Donald Trump,” according to the Department of Homeland Security. Another had the names of other mass shooters. Some weapons and magazines also had racial and antigay slurs written.   

The footage includes an arsenal of weapons and firearms equipment, including used target paper and shooting gloves.

Minneapolis Catholic school shooter changed name at 17

Homeland Security Secretary Kristi Noem said Wednesday that Westman was a transgender woman. The Associated Press reported Westman legally changed names at 17 from Robert to Robin, and listed gender identity as the reason.

The documents were filed by Westman’s mother and were signed by a judge on Jan. 15, 2020.

On Wednesday, Frey said the attention should be on the victims.

“I have heard about a whole lot of hate that’s being directed at our trans community. Anybody who is using this as an opportunity to villainize our trans community or any other community out there, has lost their sense of common humanity. Kids died. This needs to be about them,” he said.

Robin Westman was former student at Annunciation Catholic School: Police

Minneapolis Police Chief Brian O’Hara confirmed Thursday that Westman was a former student at Annunciation Catholic School.

During a Thursday appearance on NBC, O’Hara also said Westman’s mother previously worked at the parish.

Police say Robin Westman died outside church

O’Hara said Westman died of a self-inflicted gunshot wound in a parking lot outside the church.

“During the Mass, a gunman approached on the outside on the side of building and began firing a rifle through the church windows towards the children sitting in pews at the mass,” O’Hara said. “The sheer cruelty and cowardness of firing into a church full of children is absolutely incomprehensible.”

O’Hara said it appeared that most, if not all, of the shooting occurred outside the building. He also noted that the shooter had placed a 2-by-4 on the outside of a church door.

Robin Westman had no criminal record

O’Hara said Westman, dressed in black, was armed with a rifle, a shotgun and a pistol. He added that it was unclear if Westman, who does not have a known criminal history, was a former student of the school or an employee.

“We are looking through information left behind to try and determine some type of motive,” O’Hara said.

What we know about the Annunciation School in Minneapolis

O’Hara said 19 people were shot, including two children, ages 8 and 10, who died at the scene. 

Other victims ranged from ages 6 to 15, along with several elderly adults attending the church service. 

Classes began Monday for the school, which serves pre-kindergarten through eighth grade. The school was closed Thursday.