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.