Mr. Gendron pleaded not guilty to those charges and is being held without bail.
The F.B.I. is still investigating the case, including whether any other white supremacists or participants in online chats knew of Mr. Gendron’s plans or played a role in inciting him, officials said.
The attack at Tops, and an even more deadly massacre at an elementary school in Uvalde, Texas, has already impacted both policy and politics nationally and in New York. The State Legislature passed a series of new laws to tighten restrictions on gun ownership, including raising the minimum age to buy a semiautomatic rifle to 21. And Gov. Kathy Hochul, a Democrat, issued a pair of executive orders, one of which is aimed at enhancing the monitoring of online extremism by the State Police.
The mass shooting in Buffalo also led Representative Chris Jacobs, a Republican who represents some of the city’s suburbs, to embrace a series of gun control measures, prompting a furious blowback from members of his own party. As a response, Mr. Jacobs said he would not run for a new term in November, but decried many Republicans’ unblinking opposition to any gun control.
On Sunday, Senate leaders in Washington said that they had reached a framework for a bipartisan deal on a series of modest reforms, including enhanced background checks for prospective gun buyers under the age of 21 and funding for so-called red-flag laws that allow authorities to confiscate guns from people deemed to be dangerous to themselves or others.
In their comments on Wednesday, the Justice Department officials took pains to emphasize that their actions in Buffalo were not focused on a single act of racial violence, but part of a larger effort to combat white supremacy.
“This process may not be as fast as some would hope,” said Trini E. Ross, the United States Attorney for the Western District of New York, on Wednesday in Buffalo. “But it will be thorough, it will be fair, it will be comprehensive.”
Reporting was contributed by Dan Higgins from Buffalo and Benjamin Weiser.