Payton S. Gendron allegedly traveled more than three hours from Conklin, New York, to the Tops Friendly Market, according to law enforcement, to carry out the attack in a predominantly Black community.
He was wearing military fatigues, body armor and a tactical helmet when he shot four people in the parking lot of the Tops supermarket around 2:30 p.m. and then allegedly shot nine people inside before surrendering to authorities.
He was confronted by a retired Buffalo police officer working security who shot the suspect but without effect due to the suspect's body armor, police said.
Grendon was arraigned on one count of first-degree murder to which he pleaded not guilty. He has been ordered to be held without bail, according to the Eric County District Attorney's office.
The district attorney also revealed a copy cat threat, after a 52-year-old man was arrested for threatening to shoot up a Buffalo pizzeria and a brewery.
Suspected shooter visited Buffalo in March before supermarket attack
A new document obtained by ABC News appears to show how Gendron carefully planned out his attack at least two months before he allegedly shot and killed 10 people at a Tops Friendly Market in Buffalo.
According to the document, Gendron, on March 8, drove to Buffalo and visited the Tops supermarket and was confronted by a security guard at the store as he was compiling detailed plans of the location.
Police confirmed his visit in a press conference on Monday afternoon.
"The individual was here in early March," Buffalo Police Commissioner Joseph Gramaglia said, citing evidence developed during the course of the investigation.
Suspect spotted day before
Shonnell Harris Teague, an Operations Manager at Tops, said she saw the alleged shooter Payton Gendron sitting on a bench outside of her Tops store the day before the shooting on Friday around 1:30 p.m. He sat outside the store for several hours dressed in the same camouflage outfit with a camper bag on his back, she said.
"He had the same clothes on, but he had camper gear on his back like he was asking people for change," Teague said.
Gendron entered the store several hours later in the evening, and appeared as if he was bothering customers, she said. Teague asked him to leave, and he did so without an argument.
"I asked him can he please leave the store - you're gonna have to stand outside," Teague said. He left without an argument, she added.
The next time Teague saw him, was on Saturday in the midst of the mass shooting at her store . She had escaped out of Tops' back receiving doors when she saw Gendron.
"I see him with his gear on and his gun and how it was all strapped on. And he shot a man that was already, I don't know if the man was moving. He must have shot him again."
"I seen all the other bodies on the ground...it was just a nightmare."
Law enforcement sources told ABC News the alleged shooter's extremist beliefs may have been cultivated online and he appears to have expressed racially motivated extremist views in his online postings.
WATCH | ABC News Special Report on Buffalo shooting
A 180-page document believed to have been posted on the internet by the suspect, is a hate-filled screed fixated on the notion of "replacement theory," a white supremacist belief that non-whites will eventually replace white people because they have higher birth rates, authorities said.
Gendron, the purported author of the document, espouses racist and anti-Semitic tropes throughout the document, which he appears to have posted before he carried out the alleged attack, according to authorities.
Among the posts that investigators are looking at include online writings in which the suspect praises other mass shooters who were also motivated by racist ideology, including South Carolina church shooter Dylann Roof, the New Zealand mosque shooter Brenton Tarrant and the Pittsburgh Tree of Life synagogue shooter Robert Gregory Bowers.
In the document, the suspect also appears to outline a plan for his alleged attack, including time and place, and writes that he chose this location because there is a high concentration of Black people in the area.
Neighbors of the Gendron family told ABC News that the suspect is a former student at Broome Community College, part of the State University of New York college system -- a detail confirmed by a spokesperson for the school.
Police in Broome County, New York, were called by a local high school in June 2021 after they reported that Gendron threatened a shooting at graduation or during that time, law enforcement sources familiar with the matter confirmed to ABC News. Following a police investigation, no charges were filed against Gendron, who received a mental health evaluation and counseling after the incident.
A home associated with Gendron was searched by the FBI and New York state police, law enforcement officials and eyewitnesses confirmed to ABC News.
New York Gov. Kathy Hochul told Eyewitness News reporter Derick Waller, that Gendron legally purchased the AR-15 assault-style rifle that was allegedly used in the supermarket shooting at a gun store in his home county of Broome.
But the "legally obtained weapon" was modified and became "illegal," Hochul said.
"It's mostly the illegal guns and magazine capacity enhancements that are causing a lot of problems in New York City and all the way here to Buffalo," she added.
Gendron was arraigned on Saturday evening before the Buffalo City Court on one count of first-degree murder, according to a statement from the Erie County District Attorney's office.
The suspect entered a plea of not guilty. If convicted, he faces a maximum sentence of life in prison without the possibility of parole, according to the DA's office.
But according to Erie County District Attorney John Flynn, further charges against Gendron are possible.
"My office is working closely with the U.S. Attorney's Office and our partners in law enforcement into potential terrorism and hate crimes. This is an active investigation and additional charges may be filed," Flynn said in a statement.
The U.S. Department of Justice is also investigating the shooting as "a hate crime and an act of racially motivated violent extremism," Attorney General Merrick 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," Garland said.
Gendron's next court hearing is set for May 19 and will remain in custody, where he is ordered to be held without bail, according to the DA's office.
ABC News' Aaron Katersky, Bill Hutchison, John Santucci, Laura Romero and Olivia Rubin contributed to this report.
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