The shooter, identified as 23-year-old Robin Westman, fired 116 rifle rounds through stained-glass windows while the children celebrated Mass during the first week of classes at the Annunciation Catholic School, Minneapolis police chief Brian O'Hara said.
"It is very clear that this shooter had the intention to terrorise those innocent children," O'Hara said.
Acting US Attorney Joe Thompson said videos and writings the shooter left behind show that the shooter "expressed hate towards almost every group imaginable".
The only group Westman did not hate was "mass murderers and shooters," Thompson said.
Investigators recovered hundreds of pieces of evidence from the church and three residences, the police chief said.
They found more writings from the suspect but no additional firearms or a clear motive for the attack on the church the shooter once attended.
"No evidence will ever be able to make sense of such an unthinkable tragedy," O'Hara said.
Two children, aged eight and 10, died in the shooting.
City officials on Thursday increased to 15 the number of wounded children - aged six to 15 - in addition to three parishioners in their 80s who were also injured.
One child was in critical condition on Thursday while 11 other victims remained in hospitals.
Westman, whose mother worked for the parish before retiring in 2021, left behind several videos and page upon page of writings describing a litany of grievances.
One read: "I know this is wrong but I can't seem to stop myself."
O'Hara said Westman was armed with a rifle, shotgun and pistol and died by taking his own life.
On a YouTube channel, videos that police say may have been posted by the shooter show weapons and ammunition, and list the names of mass shooters.
What appears to be a suicide note to family contains a confession of long-held plans to carry out a shooting and talk of being deeply depressed.
Reverend Dennis Zehren, who was inside the church with the nearly 200 children, said the responsorial psalm - which spoke of light in the darkness - had almost ended when he heard someone yell, "Down down, everybody down," and gunshots rang out.
FBI Director Kash Patel said on X that the attack was an act of domestic terrorism motivated by hate-filled ideology, citing the shooter's statements against multiple religions and calls for violence against US President Donald Trump.
Minnesota governor Tim Walz on Thursday sent state law enforcement officers to schools and churches in Minneapolis, saying no child should go to school worried about losing a classmate or gunshots erupting during prayer.
On a YouTube channel titled Robin W, the person filming the video points to two windows in what appears to be a drawing of the church and then stabs it with a long knife.
The now-deleted videos also show weapons and ammunition, scrawled with "kill Donald Trump" and "Where is your God?" along with the names of past mass shooters.
There also were hundreds of pages written in Cyrillic, a centuries-old script still used in Slavic countries.
In one, Westman wrote, "When will it end?"
There were no past arrests or anything in the shooter's background that would have prevented Westman from being able to legally purchase a firearm, investigators said on Thursday.
Federal officials referred to Westman as transgender, and Minneapolis mayor Jacob Frey decried hatred being directed at "our transgender community".
In 2020, a judge approved a petition, signed by Westman's mother, asking for a name change from Robert to Robin, saying the petitioner "identifies as a female and wants her name to reflect that identification".
