US President Joe Biden has condemned white supremacy as "a poison running through our body politic" during a visit to Buffalo, New York.
Ten black people were killed at a supermarket in the city on Saturday in what is believed to be a racially motivated hate crime, reports BBC.
The suspect, 18, identified himself as a fascist and white nationalist in a document posted online.
Mr Biden on Tuesday said the young man belonged to "a hateful minority".
After meeting families impacted by the shooting, he eulogised the victims for their "individual lives of love, service and community that speak to the bigger story of who we are as Americans".
"White supremacy will not have the last word," he said.
Local officials say the alleged attacker drove more than 320 km (200 miles) to deliberately seek out an area with a high black population. He would have continued to target other such areas if he had not been stopped, according to investigators.
In his so-called manifesto, he referenced "white genocide" and "white replacement" conspiracy theories to explain his resentment towards minority groups. In the rambling screed, he also identified politically as being on the "authoritarian left".
The president accused the shooter of giving into "a hateful, perverse ideology rooted in fear and racism".
Making mention of the deadly far-right extremist rally in Charlottesville, Virginia in 2017 - the incident he says persuaded him to run for office again - he warned that America's democracy was "in danger like it hasn't been in my lifetime".
"Hate and fear have been given too much oxygen by those who pretend to love America but don't understand America," Mr Biden said.
"I call on all Americans to reject the lie and I condemn those who spread the lie for power, political gain and profit. We cannot remain silent."
Mr Biden held individual meetings with grieving families and also visited the makeshift memorial outside the Tops grocery store that was the site of Saturday's rampage.
Buffalo Mayor Byron Brown told media that the president had "seemed very moved by what he saw in the community" and he was hopeful for more action on gun control.
The president was joined on Tuesday's trip by First Lady Jill Biden, New York's Democratic senators and other top officials.