The author of the diary posted hand-drawn maps of the grocery store in Buffalo, New York, along with tallies of the number of black people he counted there, and recounted how a black security guard confronted him that day to ask what he was up to. A black security guard was among the dead in Saturday's rampage.
The diary taken from the chat platform Discord came to light two days after Payton Gendron, 18, allegedly opened fire with an AR-15-style rifle at the Tops Friendly Market. He was wearing body armour and used a helmet camera to live-stream the bloodbath on the internet, authorities said.
He surrendered at the supermarket and was arraigned on a murder charge. He pleaded not guilty. Federal authorities are contemplating bringing hate crime charges.
Copies of the online materials were shared with the Associated Press by Marc-Andre Argentino, a research fellow at the London-based International Centre for the Study of Radicalisation and Political Violence.
A transcript of the diary entries was apparently posted publicly before the attack. The FBI's top agent in Buffalo, Stephen Belongia, indicated that investigators were looking at Gendron's Discord activity, citing posts last summer about body armour and guns, and others last month in which he taunted federal authorities.
Messages seeking comment were left with Gendron's lawyers.
The violence spread grief and anger in Buffalo and beyond.
Former Buffalo fire commissioner Garnell Whitfield Jr, who lost his 86-year-old mother, Ruth Whitfield, in the shooting, asked how the country could allow its history of racist killings to repeat itself.
"We're not just hurting. We're angry," Whitfield said at a news conference. "We treat people with decency, and we love even our enemies."
"And you expect us to keep doing this over and over and over again, over again, forgive and forget," he said. "While people we elect and trust in offices around this country do their best not to protect us, not to consider us equal."
The entries detail a March 8 reconnaissance visit the writer made to Buffalo, more than 300 kilometres from Gendron's home in Conklin, New York.
Buffalo Police Commissioner Joseph Gramaglia said at a news conference there was information indicating Gendron was in Buffalo in March, but declined to say more.
At the White House, President Joe Biden, who planned a visit Tuesday to Buffalo, paid tribute to the slain security guard, retired police officer Aaron Salter.
Salter fired repeatedly at the attacker before being shot and killed. Biden said Salter "gave his life trying to save others".
Along with the 10 black people killed, three people were wounded: one black, two white.
