Investigators say that the attacker who opened fire on pupils as they were praying at a church in Minneapolis was "obsessed with the idea of killing children".
Robin Westman, who killed two children and injured 18 others, did not seem to have any specific motive, according to Minneapolis Police Chief Brian O'Hara.
The attacker "appeared to hate all of us", the chief said on Thursday, adding: "More than anything, the shooter wanted to kill children".
The murdered children have been identified by family as Fletcher Merkel, eight, and Harper Moyski, 10.
"Yesterday, a coward decided to take our eight-year-old son Fletcher away from us," his father, Jesse Merkel, told reporters.
"Fletcher loved his family, friends, fishing, cooking and any sport that he was allowed to play," he said.
"Give your kids an extra hug and kiss today. We love you, Fletcher. You'll always be with us," he continued, choking back tears.
The parents of Harper Moyski, Michael Moyski and Jackie Flavin, said in a statement that their daughter "was a bright, joyful, and deeply loved 10-year-old whose laughter, kindness, and spirit touched everyone who knew her".
"As a family, we are shattered, and words cannot capture the depth of our pain," the said, adding that they hope "her memory fuels action" to stop gun violence.
"No family should ever have to endure this kind of pain.... Change is possible, and it is necessary - so that Harper's story does not become yet another in a long line of tragedies."
Officials have released few details so far about the suspect's background, but say Westman previously attended the church's school and had a mother who had worked there.
The 23-year-old suspect is believed to have approached the side of the Annunciation Church, which also houses a school, and fired dozens of shots through the windows using three firearms. Police also found a smoke bomb at the scene.
Witnesses have described seeing children bleeding as they fled from the church, begging for help from strangers.
In a news conference on Thursday, acting US Attorney General for Minnesota Joseph Thompson said "the shooter expressed hate towards many groups, including the Jewish community and towards President Trump".
The attacker, who died at the scene of a self-inflicted gunshot wound, left a note, officials said, but they added that a definitive motive may never be known.
"I won't dignify the attacker's words by repeating them, they are horrific and vile," said Mr Thompson.
Westman's name was legally changed from Robert to Robin in 2020, with the judge writing: "Minor child identifies as a female". However, some federal officials and police have referred to Westman as a man when discussing the attack.
Chief O'Hara told reporters that news outlets should stop using the killer's name, because "the purpose of the shooter's actions was to obtain notoriety".
He added that she, "like so many other mass shooters that we have seen in this country too often and around the world, had some deranged fascination with previous mass shootings".
US officials have warned for years that mass shooting can lead to copycat killings, as killers seek to become famous through their heinous crimes.
Several major news organisations have a policy of not identifying mass killers.
