CNN  — 

The 18-year-old White man accused of killing 10 people in a racist mass shooting Saturday at a supermarket in Buffalo, New York, had visited the area in early March, police commissioner Joseph Gramaglia said Monday.

“We found some things that show that he was here in early March, and then again, we know he was here on Friday, basically doing reconnaissance on the area,” Gramaglia told CNN’s Erin Burnett. “He was in the store, both on Friday and Saturday.”

The suspect, who is from the city of Conklin, about 200 miles away, opened fire Saturday at the Tops Friendly Markets store in a predominantly Black neighborhood, shooting 13 people before surrendering to police.

The massacre follows other mass shootings in recent years in which authorities say a White supremacist suspect was motivated by racial hatred, including in El Paso, Texas, Charleston, South Carolina, and as far as Norway and New Zealand.

The commissioner said he couldn’t comment on whether the suspect was at the store in March but in social media posts the accused shooter revealed he had been to the store March 8 and spent months planning his attack.

The suspect, Payton S. Gendron, wrote in posts on Discord that were shared on the hate-filled online forum 4Chan that he went into the store at noon, 2 p.m. and 4 p.m. during his March visit. He wrote that on his way from his home in Broome County he got a speeding ticket.

Gendron noted in his post the activity taking place inside the market, like how many Black and White people were there. He also drew a map depicting the store aisles, pharmacy, bakery and exit points of the building.

A woman chalks a message Sunday at a makeshift memorial outside of a Tops grocery store in Buffalo.

Gendron wrote that as he did his last reconnaissance visit he was approached by a “Black armed security guard” who said, “I’ve seen you go in and out… What are you doing?” The suspect wrote that he told the security guard that he is collecting “co