MINNEAPOLIS, United States — A shooter who sprayed bullets into a church filled with young students was "obsessed with the idea of killing children," investigators said Thursday, after reviewing writings and videos that detailed hate for a myriad of groups.

As young children attended Mass on Wednesday marking their first week back at school, the attacker opened fire through the stained-glass windows of a church in the Midwestern city of Minneapolis, in the latest deadly shooting that has shaken the United States.

The assailant, who died by suicide in the parking lot, left behind a manifesto, online videos and hundreds of pages of writings that investigators have been sifting through while searching for a motive.

"The shooter expressed hate towards almost every group imaginable," including Mexicans, Christians and Jews, acting US attorney for Minnesota Joseph Thompson told a press conference.

"The shooter's heart was full of hate."

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The one group the attacker did not hate was "the most notorious school shooters and mass murderers in our country's history," whom the suspect "idolized," Thompson said.

In particular, "the shooter was obsessed with the idea of killing children."

The FBI has gathered evidence "demonstrating this was an act of domestic terrorism motivated by a hate-filled ideology," director Kash Patel said on X earlier Thursday.

Two children, aged eight and 10, were killed in the pews of the church during the attack.

The number of wounded children rose to 15 on Thursday after one previously unknown victim came forward, city police chief Brian O'Hara told the press conference. Three people in their 80s were also injured.

A child is in critical condition, and a pensioner is in serious condition at Hennepin Healthcare facilities, CEO Thomas Klemond told reporters.

Police found 116 rifle rounds and three shotgun shells at the scene, as well as a round that appeared to have been stuck in the chamber of a handgun, O'Hara said.

The shooter, who had no criminal record, had recently purchased all three guns legally, according to police.

The attacker was a 23-year-old who legally changed names in 2020 and identified as a transgender woman, authorities have said.

US church shooter 'obsessed with idea of killing children'

O'Hara requested the media stop saying the shooter's name because the purpose of this "heinous attack" was to gain "notoriety."

The shooter once attended the Catholic school linked to the church, he added, and the attacker's mother also previously worked at the church.