BUFFALO, NEW YORK — The white teen accused of murdering 10 Black people at a New York grocery store planned the shooting for months — and scoped out the location ahead of time, social media posts attributed to the suspect show.

Payton Gendron, 18, visited the Tops Friendly Market in Buffalo three times on March 8, and wrote afterwards of his intention to commit murder — according to posts he is believed to have made on Discord and 4chan, both sites popular with radicals.

He wrote about how many Black and white shoppers were inside and mapped out the supermarket’s aisles, CNN and The Washington Post reported.

Gendron posted that during his third trip he was approached by a “Black armed security guard” who asked him what he was doing going in and out the store.

The alleged shooter made up an excuse about collecting census data. “In hindsight that was a close call,” Gendron wrote.

In a post on March 10, Gendron wrote, “I’m going to have to kill that security guard at Tops. I hope he doesn’t kill me or even hurt me instantly,” CNN reported.

Among the victims of Saturday’s shooting was a retired police officer working as a security guard. He fired several shots at the assailant before being shot himself, police said.

Gendron first wrote about killing Black people in December and decided in February to target Tops based on its large surrounding African American population, according to the Post, which reviewed more than 600 pages of online messages.

Gendron said he had chosen Buffalo as the preferred site of his attack because its zip code had a higher Black population than other locations he was considering, CNN reported.

He allegedly wrote about how he planned to carry out his attack on March 15 but delayed several times.

Wearing heavy body armor and wielding an AR-15 assault rifle, the white supremacist is accused of livestreaming Saturday’s racist rampage.

He has pleaded not guilty to a single count of first-degree murder.