(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.
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 "consensus data," for which the security guard said he needed to talk to the manager.
"I asked for his name and he told me and I instantly forgot, then I said bye and thanks and walked back to my car," Gendron wrote. "In hindsight that was a close call."
In a post Gendron wrote March 10, "I'm going to have to kill that security guard at Tops I hope he doesn't kill me or even hurt me instantly."
He added that the attack would take place March 15 but he ended up postponing the date several times.
Gendron also considered attacking a church or an elementary school before settling on a supermarket, he wrote.
The information comes as investigators have dug into a 180-page diatribe posted online and attributed to the suspected gunman that lays out in detail his motives and plans for the attack.
Gendron pleaded not guilty Saturday night to a charge of first-degree murder. More charges are expected. He is in custody without bail and under suicide watch, officials said.
The suspect wore tactical gear for protection and livestreamed the attack on Twitch, authorities said. The company said it took down the video within minutes, but social media sites have struggled to stop its spread.
Inside and outside the store, he allegedly used a semi-automatic rifle to kill 10 people and wound three more, one of whom remained hospitalized Monday morning. Eleven victims were Black, officials said, and the attack is being investigated as a hate crime.
After the shooting, he exited the building and surrendered to police. Police said Monday he had plans to kill more Black people if police hadn't arrived.
"There was evidence that was uncovered that he had plans, had he gotten out of here, to continue his rampage and continue shooting people," Gramaglia told CNN. "He'd even spoken about possibly going to another store."
The suspect had other "target locations" down the street from the supermarket, according to Erie County Sheriff John Garcia. Authorities found another rifle and a shotgun in his vehicle, the sheriff told CNN's Wolf Blitzer.
"He was not going to be finished with killing as many Blacks as possible," Garcia said. He credited the quick arrival of two police officers for preventing other attacks.
Those slain range in age from 32 to 86, police said, among them the former police officer who tried to stop the gunman and a number of people doing their regular weekend grocery shopping.
Buffalo Mayor Byron Brown on Monday praised the guard's heroism, as well as the quick police response.
"Many more people would probably have been killed and injured if the Buffalo Police did not get to the scene as quickly," Brown said. "They were able to subdue the gunman, they were able to take him into custody without incident and protect the surrounding neighborhood."
The suspect has a court hearing Thursday, Erie County District Attorney John Flynn said. He also said a request for a mental health forensic examination has been withdrawn by the suspect's attorney.
Since the shooting, officials have looked at what they say was the suspect's racist intent and his history.
"We continue to investigate this case as a hate crime, a federal hate crime, and as a crime perpetrated by a racially motivated, violent extremist," Stephen Belongia, special agent in charge of the FBI Buffalo field office, said Sunday at a news conference.
Federal prosecutors are expected to bring charges against the suspect in the coming days, law enforcement officials told CNN on Monday.
The 180-page document attributed to Gendron and posted online before the shooting lays out the alleged shooter's motives and shows the meticulous planning that went into the massacre. CNN independently obtained the document shortly after the mass shooting -- before authorities released the name of the suspect -- and law enforcement sources have told CNN its description of guns matches the weapons the suspect used.
In it, the suspect allegedly detailed how he had been radicalized by reading the online message board 4chan and described himself as a White supremacist, fascist and anti-Semite. He subscribed to a "great replacement" theory, or the false belief that White Americans are being "replaced" by people of other races. Once a fringe idea, replacement theory has recently become a talking point for Fox News' host Tucker Carlson as well as other prominent conservatives.
The suspect also wrote he was inspired by the 2019 mass killing at two mosques in Christchurch, New Zealand, in which the gunman similarly wrote a lengthy document and livestreamed the attack.
The Buffalo suspect wrote he began seriously planning the attack in January. The document's author also writes that he targeted this Buffalo neighborhood because it's in a ZIP code that "has the highest Black percentage that is close enough to where I live."
Indeed, the ZIP code that includes the store, 14208, is 78% Black, the highest percentage of a Black population in any ZIP code in upstate New York, according to the US Census Bureau's 2020 American Community Survey. The shooting suspect is from the town of Conklin, New York, a three-and-a-half-hour drive from Buffalo near the Pennsylvania border.
The document also states the suspect bought the main gun he used, a Bushmaster XM-15, from a gun store before "illegally modifying it."
"We are obviously going through (the document) with a fine-toothed comb and reviewing that for all evidence," prosecutor Flynn told CNN.
CNN has obtained video recorded inside the store showing the alleged gunman pointing his rifle at a person on the ground but not shooting him.
The video is taken from the point of view of the shooter, after he's fired at several people. In it, the shooter turns the weapon on a man who is curled up on the ground near what looks like a checkout lane.
"No!" the man on the ground shouts.
The shooter says, "Sorry," and then turns away and continues walking down the aisle of cash registers.
The video ends at this point and it is unknown what happened next.
It's not clear why the man was apparently spared or why the gunman says "sorry."
A year ago, the suspect landed on the radar of police as a student at Susquehanna Valley High School, officials said.
He made an "ominous" reference to murder-suicide through a virtual learning platform in June, the Susquehanna Valley Central School District said Monday. Though the threat was not specific and did not involve other students, the instructor immediately informed an administrator who escalated the matter to New York State Police, a spokesperson told CNN, adding the law limits what more school officials can say.
Concern arose after the suspect turned in a high school project about murder-suicides, leading to a state police investigation, said Garcia, the sheriff.
"The state police arrived at his house at that point last year," he said. "He stayed at a facility -- I'm not sure if it was a hospital or a mental health facility -- for a day and a half."
State police investigated an unnamed 17-year-old student who had made "a threatening statement" in June at the high school, they confirmed. The student was taken into custody and to a hospital for a mental health evaluation.
It was not the sort of involuntary commitment that would have precluded the suspect from purchasing a weapon, state police spokesperson Beau Duffy said.
Correction: An earlier version of this story misstated the range in ages of those killed in the Buffalo supermarket shooting. They were between 32 and 86 years old.