The man suspected of shooting dead 10 people at a supermarket in Buffalo, New York, deliberately sought a site with a high black population, authorities say.
The suspect, Payton Gendron, 18, drove more than 320km (200 miles) to carry out the attack, police say.
The attack is being investigated as an act of racially motivated violent extremism.
Buffalo Mayor Byron Brown said the suspect arrived intending to take "as many black lives as possible".
A 180-page document seemingly authored by Mr Gendron has emerged, in which he describes himself as a fascist and a white supremacist.
Questions are being asked about how he was able to carry out the attack when concerns had already been raised. "I want to know what people knew and when they knew it," New York Governor Kathy Hochul told ABC News.
Joseph Gramaglia, Buffalo's chief of police, told reporters on Sunday Mr Gendron had made "generalised threats" while still at high school. He spent a day and a half in hospital undergoing a mental health evaluation, but was then released.
He does not appear to have remained under watch by authorities.
FBI Special Agent Steven Belongia told the New York Times neither state police nor the FBI had any intelligence on Mr Gendron. Meanwhile, the gun store owner who sold him a semi-automatic weapon told several US outlets that no alert came up when his name was run through a government background check system.
Meanwhile New York's Attorney General Letitia James said her office would focus on extremist material online.
"This event was committed by a sick, demented individual who was fuelled [by a] daily diet of hate," she said.
The shooting has stunned the local community. One of those attending a vigil on Sunday told Reuters: "It just hurts, why somebody would do that."
Of the 13 people shot, police said 11 were black. Among those reported killed were a man buying cupcakes for his son's birthday and a woman who had gone shopping after visiting her husband at a nursing home.
By Mike Wendling, BBC Trending
Christchurch, El Paso, Pittsburgh and now Buffalo - all places where racially-motivated assailants, radicalised online, have taken their ideology to deadly extremes.
The gunman in Buffalo, like ones before, live-streamed his violent rampage and left a so-called "manifesto" online. It details his extremist beliefs and is packed with cherry-picked statistics, conspiracy theories and internet memes.
The file contains reams of racist and anti-Semitic sludge along with straightforward admissions that the author is a fascist and a white supremacist.
If the author can be believed - as the document also clearly contains disinformation and attempts to trick reporters into reporting false stories - he was radicalised early on during the Covid pandemic, on extremist websites and message boards.
Like after the 2019 mosque shootings in Christchurch, big social media companies will struggle to remove footage of the attack.
And surely the American debate over gun control will be reignited, however briefly.
But the underlying problem seems as intractable as ever: a worldwide network of young violent extremists, some of whom are motivated to launch deadly attacks against innocent people.
The attacker, dressed in military gear, drove into the car park at Tops Friendly Market in Buffalo - an area in which police said he had done "reconnaissance" the day before - at about 14:30 EST (19:30 BST) and began livestreaming the rampage.
A security guard fired several shots back but the gunman's bulletproof vest stopped one that hit him, police said. He then killed the guard and stalked through the store firing at other people.
Witnesses described horrific scenes. "It's like a nightmare... you see this on TV, you hear about it on TV... but I never thought I would be one of them," said one.
Mr Gendron was arrested after the attack and has pleaded not guilty to murder charges.
US President Joe Biden said facts were still being established but strongly condemned racist extremism. "We must all work together to address the hate that remains a stain on the soul of America," he said.
In a later statement, the White House announced that Mr Biden and the First Lady would travel to Buffalo on Tuesday to meet with the community.
Saturday's attack is thought to be the worst mass shooting so far in the US in 2022. Some 40,000 deaths a year involve firearms in America, a figure that includes suicides - and mass shooting events occur frequently.
Less than a day after the attack in New York state, police in southern California said one person had been killed and five injured in a shooting at a church in Orange County.
A man in his sixties was arrested after some of the churchgoers tackled him and tied him up by the ankles using an extension cord, a local sheriff said.