According to the document, Gendron, on March 8, drove to Buffalo and visited the supermarket, where he was questioned by a security guard at the store as he was compiling detailed plans of the location.
The 589-page document, which is separate from the 180-page hate-filled screed Gendron is alleged to have posted online just before the massacre, includes sketches of the supermarket, including the makeup of different aisles, with notes on how to navigate around quickly.
Law enforcement officials are investigating the document’s origins and authenticity as they try to determine who may have had access to it.
Sources tell ABC News that the document is a compilation of messages posted to the online community platform Discord starting in 2021.
According to the Washington Post, which was first to report on the document, it was uploaded to the file-sharing site MediaFire by an anonymous user on April 29, and was accessible until Monday. The document was deleted shortly after the Post contacted the platform for comment, the Post said.
On a conference call Tuesday with state and local partners, law enforcement officials said that Gendron began posting threads to Discord regarding body armor in the summer of 2021, according to a source familiar with the phone call. In April of 2022, the threads also taunted federal law enforcement, officials on the call said.
ABC News Consultant and former Department of Homeland Security official John Cohen said Discord is commonly used by high school kids and gamers.
“Because the creator of a chat group has control over content, it has also increasingly become a platform of choice for violent extremists,” Cohen said.
It is unclear who, if anyone, had access to these posts.
The document details Gendron’s interactions with what he describes as a black armed security guard during his March visit to the supermarket.
According to authorities, a security guard was killed during Saturday’s attack after firing at Gendron, who was protected by body armor.
Another post describes the Tops supermarket as the first location to be targeted, and goes on to list two other nearby locations to possibly attack, including a deli and a barbershop.
In a bulletin published Monday by the New York Police Department and obtained by ABC News, officials said this type of online communication “underscores that an online connection to an extremist culture and ideology through social media, online gaming platforms, or anonymous message boards like 4chan, can be equally effective in mobilizing individuals to violence as connections to real-world groups.”
“Online gaming platforms, in particular Twitch and Discord, have become popular social media alternatives for far-right extremists due to minimal content moderation, as opposed to more mainstream social media platforms,” the NYPD bulletin said.
Thomas Holt, director and professor in the School of Criminal Justice at Michigan State University, told ABC News that the content in these platforms has “changed over time” and that “some groups have radicalized more than others.”
Some extremist users prefer the platforms because the rules governing content are left to individual moderators, said Holt.
“In these forums you can kind of act as you wish, and it just depends on the moderators of either the subsection or the overall site to take action — and that’s highly variable,” Holt said.