A gunman wearing military-style clothing and body armor opened fire with a rifle at a supermarket in Buffalo, N.Y., killing 10 people in a shooting officials are investigating as a racially motivated hate crime.

The alleged gunman, who livestreamed the attack online, was arraigned on a first-degree murder charge hours after he was taken into custody, law enforcement officials said.

A total of 13 people were shot at the Tops Friendly Market on Saturday afternoon, officials said at a press conference. Of the 13 victims, four were store employees, including a security guard, and the rest were customers. Eleven of the victims were Black, and two were white, said Buffalo Police Commissioner Joseph Gramaglia.

"It was straight up a racially motivated hate crime," said Erie County Sheriff John Garcia. "This was pure evil."

New York Gov. Kathy Hochul called the gunman a "white supremacist who has engaged in an act of terrorism."

A public information officer with the Erie County District Attorney's Office named the suspect as 18-year-old Payton S. Gendron, who is white. He is from Conklin, N.Y., a community located southeast of Binghamton that's a more than 3-hour drive away from Buffalo.

If convicted, a first-degree murder charge holds a sentence of life without parole, said Erie County District Attorney John Flynn during a Saturday evening briefing.

The suspect pleaded not guilty and was held without bail. He's scheduled to make a court appearance on Thursday.

Stephen Belongia, the FBI Special Agent in Charge of the Buffalo field office, said the agency is investigating the shooting as a hate crime and "an instance of racially motivated violent extremism." Federal authorities are also looking at possible terrorism charges.

How the attack unfolded

The shooting began at 2:30 p.m. Eastern time outside the supermarket, which is located in a predominantly Black neighborhood, about 3 miles north of downtown Buffalo.

The gunman opened fire with an assault-style rifle, shooting four people in the parking lot, law enforcement officials said. Three of those people died.

After the gunman worked his way into the store, the security guard, a retired police officer, confronted the shooter. The suspect then shot and killed the security guard.

The suspect was later confronted by police at the front of the store. He briefly held a gun to his neck, but police said they talked him into dropping his guns and surrendering.

A racist screed posted online detailed the plan of attack

A screed authored by someone using the same name as the shooter detailed a plan for the attack. Posted to the anonymous message board 4chan, an author identifying himself as Payton Gendron says "extreme boredom" during the pandemic led to his radicalization on 4chan.

The 180-page document is full of racist rants and appears to embrace "the great replacement" white supremacist conspiracy theory that claims that an elite cabal of Jews, corporate leaders and politicians are intentionally diluting the white population through permissive immigration and by promoting diversity.

The same hateful conspiracy theory was championed by the gunman who perpetrated the massacre of 51 people in New Zealand mosques in 2019. The document's author calls the New Zealand shooter his biggest source of inspiration.

The document's author claims to be a student at the State University of New York's Broome Community College. The college said in a statement to NPR that he is not currently enrolled at the school.

The attack was livestreamed online

The shooter livestreamed the incident on the platform Twitch, according to a spokeswoman for the company. Twitch said the stream was taken offline less than two minutes after the violence started, and has indefinitely suspended the user from the service.

In remarks after the shooting, Hochul said social media companies bear some responsibility when extremists use their platforms to amplify violence.

"The social media platforms that profit from their existence need to be responsible for monitoring and having surveillance, knowing that they can be in a sense an accomplice to a crime like this, perhaps not legally but morally," the governor said.

Under federal law, online platforms have a legal shield from being held responsible for what users post. Yet there are exceptions, like when content violates federal criminal laws.

The streaming platform did not say how many viewers the livestream received during the brief time it was available, but the company said it is monitoring the platform for restreams of any parts of the graphic footage, which violates its rules against streaming violence.

"We are devastated to hear about the shooting that took place this afternoon in Buffalo, New York. Our hearts go out to the community impacted by this tragedy," said Twitch spokeswoman Samantha Faught.

The Biden administration responds

The White House said President Biden had been briefed on the shooting and will continue to receive updates.

"Tonight, we grieve for the families of ten people whose lives were senselessly taken and everyone who is suffering the physical and emotional wounds of this horrific shooting," Biden said in a statement late Saturday.

The president said the investigation was still ongoing into the attacker's motivation, but said a racially motivated hate crime "is abhorrent to the very fabric of this nation. Any act of domestic terrorism, including an act perpetrated in the name of a repugnant white nationalist ideology, is antithetical to everything we stand for in America. Hate must have no safe harbor. We must do everything in our power to end hate-fueled domestic terrorism."

Attorney General Merrick Garland said the FBI and ATF were working closely with the Buffalo Police Department and other law enforcement agencies.

"The Justice Department is investigating this matter as a hate crime and an act of racially-motivated violent extremism," Garland said in a statement. "The Justice Department is committed to conducting a thorough and expeditious investigation into this shooting and to seeking justice for these innocent victims."

Speaking to NPR's All Things Considered, Buffalo Mayor Byron Brown said the investigation is ongoing and that many in Buffalo are grieving.

"Collectively, our community is heartbroken and is in pain at this point," he said. "I know a number of the victims and know a number of the families involved. Many people in our community are touched by this in some way."

The FBI have made a public request for further information.

NPR's Odette Yousef and member station WBFO contributed reporting.