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.
A 180-page document seemingly authored by Gendron 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 news agencies.
Police said the suspect had done “reconnaissance” of the area the day before the shooting.
“This event was committed by a sick, demented individual who was fuelled [by a] daily diet of hate,” she said.
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.
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.
Gendron was arrested after the attack and pleaded not guilty to murder charges.
SOURCE: NEWS AGENCIES