The white man accused of killing 10 Black people in a racist attack on a Buffalo supermarket was indicted by a grand jury on Wednesday on a state domestic terrorism and hate crime charge that would carry a mandatory sentence of life in prison.

Payton Gendron is scheduled to be arraigned on Thursday on the new, 25-count indictment, which builds on a previous murder charge hastily prepared in the hours after the May 14 shooting.

Investigators stand outside during a moment of silence for the victims of the Buffalo supermarket shooting outside the Tops Friendly Market. Photo / AP

The 18-year-old has now also been charged with the attempted murders of three people who were shot during the attack, but survived, and with using a weapon while committing a felony.

He has pleaded not guilty. Prosecutors had told a judge on May 20 the grand jury had voted to indict Gendron but did not disclose charges, saying proceedings were ongoing.

Gendron's attorney, Brian Parker, said he had not seen the indictment and could not comment, adding that prosecution and defence attorneys have been barred by a judge from discussing the case publicly.

Michael Jordan and Heather Delorm, friends of Buffalo shooting victim Roberta Drury, visit a memorial for the victims of the Buffalo supermarket shooting. Photo / AP

The horrific nature of the crime and number of victims was likely to already guarantee a life sentence if Gendron is convicted. New York has no death penalty. But adding a state terrorism charge could carry additional emotional resonance and help authorities send a message about violent extremism.

The domestic terrorism charge — Domestic Acts of Terrorism Motivated by Hate in the First Degree — accuses Gendron of killing "because of the perceived race and/or colour" of his victims.

"This man was motivated by hate against people he never met for no reason other than the colour of their skin," said Buffalo lawyer John Elmore, who represents the families of victims Katherine "Kat" Massey, 72, and Andre Mackniel, 53. Elmore said he hoped for a conviction on every count.

Former Governor Andrew Cuomo proposed the domestic terrorism hate crime law in August 2019, in the wake of a mass shooting targeting Mexicans at a Walmart store in El Paso, Texas. The measure, dubbed the "Josef Neumann Hate Crimes Domestic Terrorism Act" after an attack at a rabbi's home in Munsey, New York, was signed into law on April 3, 2020, and took effect November 1, 2020.

A group prays at the site of a memorial for the victims of the Buffalo supermarket shooting outside the Tops Friendly Market. Photo / AP

The law expanded on a previous domestic terrorism statute passed after the 9/11 terrorist attack that was largely envisioned as a way to go after international extremism.

Prosecutors said Gendron drove about three hours to Buffalo from his home in Conklin, New York, intending to kill as many Black people as possible. Shortly before the attack he posted documents that outlined his white supremacist views and revealed he had been planning the attack for months.

The gunman, carrying an AR-15-style rifle he had recently purchased, opened fire on Saturday afternoon shoppers at a Tops supermarket in Buffalo.

Murder charges were filed for each of the victims, who ranged in age from 32 to 86 and included eight customers, the store security guard and a church deacon who drove shoppers to and from the store with their groceries.

A mourner embraces Angela Crawley, left, daughter of Ruth Whitfield, a victim of the Buffalo supermarket shooting. Photo / AP

The shooting, followed 10 days later by a mass shooting that killed 19 children and two teachers inside a Uvalde, Texas, elementary school, has renewed a national debate about gun control.

Mackniel was in the Buffalo store to buy a birthday cake for his 3-year-old. Massey was a community activist who had championed gun control and fought against racism, Elmore said.

"To have her life taken away by a white supremacist extremist at the hands of a weapon of mass destruction is extremely upsetting to me," he said. He is part of a team of attorneys exploring potential legal action against the manufacturers of the weapon and body armour used by the gunman, as well as social media platforms.

Vice President Kamala Harris and her husband Doug Emhoff visit a memorial near the site of the Buffalo supermarket shooting after attending a memorial service for Ruth Whitfield. Photo / AP

The attack was livestreamed from a helmet-mounted camera.

"Somehow we're going to find justice for the Massey family, for the Mackniel family and all those others that were affected by this tragedy," Elmore said.

Federal authorities also are investigating the possibility of hate crime charges against Gendron, who apparently detailed his plans and his racist motivation in hundreds of pages of writings he posted online shortly before the shooting.

Amanda Drury, who lost her 32-year-old sister, Roberta Drury, said she is leaving it to the legal system to say what charges are appropriate in the case.

"I'm going to continue with my trust in the justice system," she said.

- AP