Buffalo, New York - The 18-year-old behind a hate-mongering massacre at a Buffalo supermarket was just getting started, and had plans to take his anti-Black attack to other parts of the city, authorities said Monday.

Payton Gendron allegedly shot 13 people at the Tops Friendly Market on Saturday afternoon, killing 10 of them.

Police said Gendron was targeting Black people, and even had the N-word painted on the barrel of the modified assault weapon he used in the deadly attack.

"We have uncovered information that if he escaped the supermarket, he had plans to continue his attack," Buffalo Police Commissioner Joseph Gramaglia told ABC News on Monday. "He had plans to continue driving down Jefferson Ave. to shoot more Black people ... possibly go to another store (or) location."

Gendron, who is white, traveled hours from his hometown of Conklin and came dressed in body armor and a tactical helmet, authorities said. He was specifically targeting Black people, according to an online manifesto in his name, and chose Buffalo since the city had the "highest Black percentage that is close enough to where I live," Gendron allegedly wrote.

Not only did Gendron use his 180-page Google Doc to spew racist venom, but he also gave detectives and anyone else who happened to read his ramblings a step-by-step game plan of his attack, complete with a detailed explanation of how he acquired his rifles and modified the murder weapon.

Gendron, who livestreamed the shooting with a camera mounted to the protective helmet he wore, planned ahead for the maximum amount of damage, officials said.

In his rambling screed, the suspect said he deliberately loaded heavier rounds to use in an initial volley to penetrate glass at the front of the supermarket where he expected a security guard would be keeping watch.

He then loaded lighter rounds deeper in the magazine so he could use them to target shoppers and other victims in the store.

Gendron apparently knew enough about ammunition to know that lighter bullets travel faster and can tear through bodies more easily and cause more bodily damage.

Gendron eventually surrendered after the shooting, talked down by two officers after putting his own gun to his neck. He was arrested at the scene, and is being held without bail.

He pleaded not guilty to first-degree murder Saturday night.

The US Justice Department is investigating the shooting as a "hate crime and an act of racially motivated violent extremism."

Gendron’s post also cited his inspirations, including Christchurch shooter Brenton Tarrant, who murdered 51 people at two New Zealand mosques in 2019; Dylann Roof, who killed nine Black people at a church in South Carolina in 2015; and Anders Breivik, who killed 77 people in Norway in 2011.

Describing himself as a fascist, a white supremacist, and an antisemite, Gendron allegedly regurgitated tropes of the "great replacement theory," which claims that white people are being marginalized and wiped out.

The gun shop owner who sold the Bushmaster XM-15 to the suspect said his customer had passed a background check and raised no red flags.

But a security guard at the supermarket thought something was amiss two months ago, when Gendron was apparently doing reconnaissance for his deadly mission.

"I’ve seen you go in and out ... What are you doing?" the guard asked Gendron on March 8, according to a separate, nearly 600-page document posted online, ostensibly by Gendron, that was obtained by The Washington Post.

He told the guard he was "collecting consensus data" and then left, noting, "In hindsight that was a close call," according to the Post.

The 589-page document was posted April 29, referring to the supermarket as "attack area 1" and describing his plan to hit two other locations and gun down more than three dozen people in all.

Meticulous maps of the Tops store layout were included in the report on the March 8 visit. The writer noted the presence of "many Blacks" at the cash registers. An accounting done by the document writer noted that there were 53 Black people and six white people inside the store at that time.

Eleven of the 13 people shot on Saturday were Black. Victims ranged in age from 20 to 86.

Among the victims was Ruth Whitfield (86). A lawyer for her family, civil rights attorney Benjamin Crump, said the mass shooting should be treated as terrorism, and Whitfield’s family was considering legal action.

He said a lawsuit could focus on the red flags that failed to ignite preventive action.

"We can’t sugarcoat it, we can’t try to explain it away by talking about mental illness," Crump said. "No. This was an act of domestic terrorism, perpetrated by a young, white supremacist."

Other victims include a retired police lieutenant, a community activist and a substitute teacher described as a "pillar of the community."