Suspected Buffalo supermarket gunman Payton Gendron pleaded not guilty during his arraignment on 25 new charges Thursday afternoon, weeks after he allegedly perpetrated a hate crime ambush at a Tops Friendly Market store that killed 10 people.

After the plea, Judge Susan Eagan ordered him remanded without bail.

"The highest charge is a domestic act of terrorism motivated by hate in the first degree," Erie County District Attorney John J. Flynn told reporters after the court appearance. "This is a relatively new charge."

He said it was the first time in the history of New York state that such a charge had been filed.

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Gendron was initially charged in Buffalo City Court with murder after the May 14 massacre. But Flynn said Thursday that prosecutors had been weighing additional charges the entire time and initially just wanted to make sure they could hold him in custody on a "foundational charge" as they investigated the case.

County prosecutors this week tacked on more than two dozen additional charges, including 10 counts of first-degree murder and 10 of second-degree murder as a hate crime, and the Justice Department has said it is also pursuing a federal case.

Buffalo shooting suspect Payton Gendron was indicted Friday in Erie County

Buffalo shooting suspect, Payton S. Gendron, who is accused of killing 10 people in a live-streamed supermarket shooting in a Black neighborhood of Buffalo, is escorted in the courtroom in shackles, in Buffalo, New York, U.S., May 19, 2022. (Brendan McDermid)

"A racist, hate-filled outsider… came to our community with the stated intent to kill as many Black people as possible, and that outsider filmed his actions on social media and put it out for the world to see," Buffalo Mayor Byron Brown said at the news briefing. "The wheels of justice are turning very swiftly."

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The new 10-page indictment, obtained by Fox News Digital, alleges that Gendron killed the victims "in whole or in substantial part because of the perceived race and/or color of such person or persons regardless of whether that belief or perception was correct."

READ THE INDICTMENT:

The other new charges Gendron faces are three counts of attempted murder as a hate crime, one count of first-degree domestic terrorism and second-degree criminal possession of a loaded "assault weapon.," according to the indictment. He faces life in prison without the possibility of parole.

In a 180-page, hate-filled essay, Gendron allegedly outlined the racist motives and plotting that went into the attack and wrote that he knew his bulletproof vest would be able to withstand shots from the security guard’s 9 mm handgun.

Buffalo shooting Tops

Police secure an area around a supermarket where several people were killed in a shooting, Saturday, May 14, 2022, in Buffalo, N.Y.  (Derek Gee/The Buffalo News via AP)

Gendron allegedly livestreamed the shooting on Twitch before the company said it disabled the feed in under two minutes. 

Gendron, who is White, pulled up to the Tops grocery store around 2:30 p.m. on Saturday, May 14, and immediately began firing off shots. He shot four people out front and charged inside, according to Buffalo Police Commissioner Joseph Gramaglia said. Three of the initial victims died.

Inside, he exchanged gunfire with Aaron Salter, a retired Buffalo police officer, who also died in the attack.

Buffalo shooting Payton Gendron Tops

Mourners have erected multiple memorials around the area of the Tops supermarket, where 10 people were killed and there others were wounded during Saturday's mass shooting. (Fox News Digital/Michael Ruiz)

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In all, he is accused of killing 10 people in the attack and wounding three others. Eleven of the victims were Black.

The Justice Department is calling the attack "a hate crime and an act of racially motivated violent extremism," and President Biden called the shooting domestic terrorism during a visit to the city last month.

Fox News’ Laura Prabucki contributed to this report.