(CNN) — Details continue to emerge about the shooting at a Buffalo, New York, supermarket Saturday and the 18-year-old who authorities say is responsible.

Thirteen people — 11 of whom are African American — were shot, according to city officials, who have said the massacre was racially motivated, pointing in part to racist writing purportedly written by the suspect. Ten were killed, while three victims suffered non-life-threatening injuries.

The suspect — identified as Payton S. Gendron, an 18-year-old White man — surrendered to police. He has pleaded not guilty to a charge of first-degree murder, per Buffalo City Court Chief Judge Craig Hannah. Erie County District Attorney John Flynn has said he expects to file additional charges.

The FBI is investigating the shooting as both a hate crime and a case of racially motivated violent extremism, said Stephen Belongia, FBI special agent in charge of the Buffalo field office.

Here’s what we know about the shooting and the investigation.

How the shooting unfolded

When the suspect arrived at the Tops Friendly Markets supermarket on Jefferson Avenue about 2:30 p.m. Saturday, he was “very heavily armed,” according to Buffalo Police Commissioner Joseph Gramaglia.

“He had tactical gear, he had a tactical helmet on, he had a camera that he was livestreaming what he was doing.”

The suspect first opened fire in the store’s parking lot, per authorities, shooting four people. Three of those people died while another is expected to survive, Gramaglia said.

The suspect went inside the store and began shooting customers, police said. A security guard inside the store who was a retired Buffalo police officer shot the suspect. But because the suspect wore heavy armor, the bullets did not have any effect, Gramaglia said.

The suspect shot and killed the security guard and continued working his way through the store, the commissioner said. When police arrived, the suspect put his gun to his neck but later dropped the gun and took off some of his gear, he added.

He surrendered and was transported to Buffalo police headquarters.

During his arrest, the suspect made very disturbing statements describing his motive and state of mind following his arrest, according to an official familiar with the investigation. The alleged shooter made it known he was targeting the Black community during the statements, according to the official.

Warrants are being obtained for suspect’s home, vehicle, social media platforms, computer, telephone, and any other digital technologies, Gramaglia said. The investigation shows Gendron acted on his own in the shooting, he added.

What we know about the victims

Of the 13 people shot, 11 are African American and two are White, said Gramaglia, the police commissioner.

Four of the people shot were store employees, he said. Three suffered “nonfatal wounds,” he said. The fourth, security guard Aaron Salter, a retired Buffalo police lieutenant, was killed as he tried to stop the suspect. Mayor Brown called him a “hero, who tried to protect people in the store, tried to save lives, and in the process, lost his own life.”

Police identified the victims, ranging in age from 20 to 85, late Sunday.

Among them was 62-year-old Geraldine Talley, who was shopping with her fiancé when gunfire erupted, her niece Lakesha Chapman told CNN. Talley was at the front of the store, Chapman said, but her fiancé had gone to get orange juice. He escaped unharmed.

Chapman’s “Auntie Gerri” was always “the life of the party,” she said. “She was just a lover. I mean she didn’t meet a stranger, and that’s why this hurts so much,” said Chapman.

Heyward Patterson, a 67-year-old taxi driver, was waiting for passengers outside the supermarket when he was gunned down, according to his nephew, Terrell Clark.

Patterson — who loved to tell jokes and sing at church — “took pride in helping people,” Clark said. “And if the person had little or no money, he would still give them a ride. He had a big heart.”

Suspect livestreamed the violence

Livestreaming platform Twitch confirmed Saturday the suspect used its platform to stream a live broadcast during the attack, saying in a statement the video was removed less than two minutes after the violence began.

The company was “devastated” to hear about the shooting, it said, and the user “has been indefinitely suspended from our service, and we are taking all appropriate action, including monitoring for any accounts rebroadcasting this content.”

Investigators examine document purportedly written by suspect

Authorities at the state, local and federal level continue to investigate the shooting and motive, but they have already said they believe it was racially motivated.

“The evidence that we have uncovered so far makes no mistake that this is an absolute racist hate crime. It will be prosecuted as a hate crime,” Gramaglia said. “This is someone who has hate in their heart, soul and mind.”

Garcia, the Erie County sheriff, similarly said the shooting was a “straight up racially motivated hate crime from somebody outside of our community,” describing it as “pure evil.”

One piece of evidence may be a 180-page document purportedly written by the suspect and posted online before the shooting. The document, independently obtained by CNN shortly after the massacre and before authorities released the name of the suspect, is allegedly written by a person claiming to be Payton Gendron confessing to the attack.

The author attributes the internet for most of his beliefs, describes himself as a fascist, a white supremacist, and an anti-Semite.

The suspect allegedly says he bought ammunition for some time but did not get serious about planning the attack until January. The author also writes about his perceptions of the dwindling size of the white population and claims of ethnic and cultural replacement of whites.

Alongside tirades about his false belief that White Americans were being “replaced” by people of other races, the document included pages upon pages listing the equipment and clothing he planned to wear, from military-style body armor down to the brand of his underwear.

The document also included a minute-by-minute outline of the suspect’s plan. The author drew a color-coded map of the interior of the store, laying out how he planned to “shoot all black people.” It’s unclear how closely the gunman’s attack followed the plan listed in the document.

When asked during Saturday’s news conference about why authorities are calling the mass shooting a hate crime, Flynn, the district attorney, said investigators have “certain pieces of evidence” that “indicate some racial animosity.”

Further investigation this weekend also revealed Gendron made a “generalized threat” while he attended Susquehanna Valley Central High School in June 2021, Gramaglia said.

Sheriff Garcia told CNN Monday the suspect was visited last year by New York State Police after he did a high school project about murder-suicides.

According to Garcia, concerns about alleged mental health issues “were brought to light” after he turned in the post-graduation project.

“The state police arrived at his house at that point last year,” Garcia said. “He stayed at a facility — I’m not sure if it was a hospital or a mental health facility — for a day and a half.”

Additionally, Gendron was in Buffalo on Friday, Gramaglia said, and authorities have determined some locations he visited ahead of the shooting, adding he did some reconnaissance at the Tops Friendly Markets store.

Suspect was radicalized by message boards

In the document, the suspect allegedly detailed how he had been radicalized by reading online message boards, while describing the attack as terrorism and himself as a White supremacist.

He wrote he had “moved farther to the right” politically over the last three years.

The suspect started browsing the message board 4chan — a hotbed for racist, sexist and White nationalist content — in May 2020 “after extreme boredom” during the early days of the coronavirus pandemic, according to the document. Posts he had read on the site made him believe “the White race is dying out,” among other racist beliefs, and led him down a rabbit hole to other extremist websites, the document states.

The conspiracy theory of a “great replacement” has been a motivator of other violent attacks, experts in extremism have said. Some forms of the theory have more recently gone mainstream in conservative news outlets and politicians.

One day while browsing 4chan, the suspect allegedly saw a video clip of the gunman who killed 51 people in New Zealand at two mosques in 2019, according to the document. That livestream “started everything you see here,” the document states.

In addition to the New Zealand massacre, the suspect was allegedly inspired by other racist mass shooters, including Dylann Roof, the gunman who killed nine Black people at a Charleston, South Carolina, church in 2015, and the assailant who killed 77 people in Norway in 2011, according to the document.

The document includes dozens of pages of racist and anti-Semitic screeds, including some language that appears to be copied from the New Zealand shooter’s own writings.