The man suspected of shooting dead 10 people at a supermarket in Buffalo, New York, deliberately sought a site with a high black population, according to the BBC.
The suspect, Payton Gendron, 18, drove more than 320 km (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”.
Questions are being asked about how he was able to carry out the attack when he was already on the radar of authorities.
Mr Gendron had previously threatened a shooting at his high school last June, a law enforcement official told the Associated Press.
A 180-page document seemingly authored by Mr Gedron has emerged, in which he describes himself as a fascist and a white supremacist.
“I want to know what people knew and when they knew it,” New York Governor Kathy Hochul told ABC News.
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 said “it just hurts, why somebody would do that”, Reuters reported.
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.
An all too familiar pattern
As in other US shootings, the suspect left a long online trail
By Mike Wendling, BBC Trending
Christchurch, El Paso, Pittsburgh and now Buffalo – are all places where racially-motivated assailants, radicalized online, have taken their ideology to deadly extremes.
The shooter 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 pathetic attempts to trick reporters into reporting false stories – he was radicalized early on during the Covid pandemic, on extremist websites and message boards such as 4chan.
Like after the 2019 mosque shootings in Christchurch, big social media companies will struggle to zap 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 at about 14:30 EST (19:30 BST) and began live-streaming 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.
Mr Gendron was arrested after the attack and pleaded not guilty to murder charges.
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.
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.
New York’s attorney general Letitia James called the attack “domestic terrorism, plain and simple”.
Saturday’s attack in Buffalo 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.