BUFFALO, N.Y. (AP) — The white man accused of killing 10 Black people at a supermarket in Buffalo appeared in court Thursday, standing silently during a brief proceeding attended by some relatives of the victims after a grand jury indicted him.

Payton Gendron, 18, wore an orange jail uniform, a mask and handcuffs. As he was led out, someone shouted “Payton, you’re a coward!” from the courtroom gallery. He is being held in jail without bail.

Assistant district attorney Gary Hackbush said the first-degree murder indictment, which covers all 10 deaths, was handed up Wednesday.

Thirteen people in all were shot Saturday at the Tops Friendly Market in a predominantly Black neighborhood of Buffalo. Authorities are continuing to investigate the possibility of hate crime and terrorism charges.

District attorney John Flynn said his office would not comment on the case while the grand jury investigation continues.

Gendron’s lawyers also declined comment, according to defense attorney Daniel DuBois.

The victims’ family members who had attended the hearing left without immediately speaking to reporters.

Gendron, 18, livestreamed the attack from a helmet camera before surrendering to police outside the grocery store. Shortly before the attack, he posted hundreds of pages of writings to online discussion groups where he detailed his plans for the assault and his racist motivation.

Investigators have been examining those documents, which included a private diary he kept on the chat platform Discord.

At his initial court appearance last week, Gendron's court-appointed lawyer entered a plea of “not guilty” on his behalf. He is due back in court June 9.

Five days after the shooting, officials announced Thursday that evidence collection at the supermarket was completed. Tops Market president John Persons pledged to open the store as soon as possible, but said he could not give a timeline.

The massacre at the Tops supermarket was unsettling even in a nation that has become almost numb to mass shootings. All but two of the 13 people shot during the attack were Black. Gendron’s online writings said he planned the assault after becoming infatuated with white supremacist ideology he encountered online.

Stephen Belongia, the FBI’s lead agent in Buffalo, said agents were still working to piece together Gendron's motives and how he came to his extremist views.

“An important part of these sorts of incidents is to tell the whole story that may not be heard in a courtroom at a later date — to understand, to the extent that you can, what the motives are of this gunman, how he became radicalized, what he was reading, where he was reading it, who he was inspired by,” Belongia said at a news briefing.

The diary said Gendron planned his attack in secret, with no outside help, but Discord confirmed Wednesday that an invitation to access his private writings was sent to a small group of people about 30 minutes before the assault began.

Some of them accepted the invitation. It was unclear how many read what he had written or logged on to view the assault live. It also wasn't clear whether anyone tried to alert law enforcement.

New York Gov. Kathy Hochul on Wednesday authorized the state’s attorney general, Letitia James, to investigate social media platforms used by Gendron to determine if they were liable for “providing a platform to plan and promote violence.”

In Buffalo on Thursday, Police Commissioner Joseph Gramaglia said social media users can also play a role by speaking up when they see people posting violent or threatening content.

“You need to out these people,” he said at a briefing. “Expose those that are putting out those types of extreme views and let us root them out.”