The FBI is investigating Saturday’s mass killing that left 10 people dead and three injured in Buffalo, New York, as a hate crime, the agency said.

FBI officials said it is “aggressively investigating” the attack as “an instance of racially motivated violent extremism.”

Authorities say Payton Gendron, a heavily armed white 18-year-old man, entered the Tops Friendly Markets store in a predominately Black neighborhood around 2:30 p.m. and unloaded a fusillade of bullets.

He identifies himself as a white supremacist.

Witnesses said Gendron wore camouflage and looked like he was “in the army.” Reports show he was clad in body armor. The shooter broadcast the onslaught on Twitch, a live-streaming platform.

Eleven of the victims were Black, and two were white.

“It was straight-up, racially motivated hate crime from somebody outside of our community, outside of the city of good neighbors, as the mayor said, coming into our community and trying to inflict that evil upon us,” Erie County Sheriff John Garcia said in a press briefing.

Gendron pleaded not guilty to first-degree felony charges Saturday night. Erie County District Attorney John J. Flynn said it is the highest murder charge in the state. Flynn said he pushed for the suspect to be arraigned Saturday night.

“We are now investigating terrorism charges, other murder charges, along with working with our partners in the federal government so that they can perhaps file charges as well. So I assure everyone in this community, justice is being done right now, and justice will be done,” Flynn said during a briefing.

Buffalo Police Commissioner Joseph Gramaglia said Gendron drove hours away from his home in Broome County, New York — more than 200 miles from Buffalo — to commit the crime.

He killed three people in the store’s parking lot before exchanging gunfire with a security guard inside, killing the retired police officer. Six of the people Gendron killed were customers, and four were employees. The assailant later surrendered to police after he “put the gun to his own neck,” according to reports.

“It’s like walking onto a horror movie, but everything is real. It is Armageddon-like,” a police official at the scene told Buffalo News. “It is so overwhelming.”

Katherine Crofton told reporters she saw the suspect shoot a woman going into the store and another putting away groceries. A dairy worker, Will G., told the Buffalo News he heard shots while restocking milk. He and others hid in the cooler.

However, some who hid near cash registers did not escape the killer’s deadly ammo, reports show.

Others jetted outside and around the store to search for their loved ones and co-workers in the chaos.

“I just heard shots. Shots and shots and shots,” Will G. said. “It sounded like things were falling over.”

Witnesses said the suspect initially ignored officers’ commands “like he wanted them to shoot him” before they took him down.

Gendron faces life without parole for the state felony murder charge. The judge Saturday ordered a forensic examination, and the suspect must return to court in five days for a felony hearing.

Law enforcement officials received a warrant to search Gendron’s home Saturday evening. Investigators are reportedly combing through a 106-page manifesto connected to the shooting that shows admiration for South Carolina church shooter Dylann Roof and the New Zealand mosque shooter Brenton Tarrant, ABC News reports.

According to reports, Gendron’s online manifesto shows he developed racist views in recent years, which grew more radical over the past three. Gendron had been “passively preparing” for the massacre for several years, The New York Times reports. He stocked up ammunition and protective gear and occasionally held target practice. In May 2020, Gendron started following anonymous chatter on “white replacement theory.”

“If there’s one thing I want you to get from these writings, it’s that White birth rates must change. Everyday the White population becomes fewer in number,” the manifesto reportedly says. “To maintain a population the people must achieve a birth rate that reaches replacement fertility levels, in the western world that is about 2.06 births per woman.”

Gendron also created a timeline for the attack, including a layout of the store.

The self-proclaimed white supremacist mapped out his parking space, selected his meal beforehand and picked the spot where he would start the live stream. He wrote down his plan to kill the security guard and shoot Black shoppers twice in the chest if he could, reports show.

New York Gov. Kathy Hochul slammed Twitch and social medial companies for not taking action against hate speech on their platforms, demanding they be held accountable for allowing “this hate to be spewed and others who are like-minded to be radicalized.”

“A community was shattered by an individual, a white supremacist, who has engaged in an act of terrorism and will be prosecuted as such in a cold-hearted, calculated way, a military-style execution targeting people who simply want to buy groceries in a neighborhood store,” Hochul said during a briefing.

Representatives for Twitch said the video was removed within two minutes.