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”.
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. He underwent a mental health examination afterwards.
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.
Police said the suspect had done “reconnaissance” of the area the day before the shooting.
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.
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 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.
Mr Gendron was arrested after the attack and has 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.
Attorney General 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. (BBC)