BUFFALO, NEW YORK (REUTERS) -  The 18-year-old man accused of the deadly mass shooting in Buffalo, New York, visited the city in March and again the day before the rampage, police said on Monday (May 16), as public figures decried the suspect’s racist ideology and the spread of white supremacy.  

The FBI said Payton Gendron, 18, who is white, carried out an act of "racially motivated violent extremism" when he opened fire with a semi-automatic rifle on Saturday at the Tops Friendly Market in a predominantly African-American neighbourhood of Buffalo. Eleven of the 13 people struck by gunfire were Black.

Ten of the victims – nine shoppers and a retired police officer working as a store security guard who exchanged gunfire with the assailant – were killed.

Gendron, who police said surrendered to officers who confronted him inside the store, has been jailed without bond on a charge of first-degree murder. He entered a plea of not guilty.

Investigators have said they are searching through phone records, computers and online postings, as well as physical evidence, as new details about the suspect's past and meticulous planning emerge.

The Washington Post reported on Monday that the suspect, a resident of Conklin, New York, near the Pennsylvania border, roughly 200 miles (320km) from Buffalo, made an "apparent reconnaissance" trip to the Tops store in March to map out its layout and location in preparation for the attack.

He was confronted there at the time by a store security guard, who thought he looked suspicious, according to the Post, citing an account of the visit that the newspaper said was posted online by an individual identifying himself as Gendron.

Buffalo Police Commissioner Joseph Gramaglia told reporters at a news briefing on Monday that the suspect had paid a visit to Buffalo in early March, but he declined to confirm other details of the investigation reported by the Washington Post or other news media.

Authorities said the suspect returned to Buffalo on Friday to undertake a final "reconnaissance" of the area.

The Post said the trip to Buffalo in March was detailed in a series of messages compiled in a 589-page document posted on an Internet messaging platform but since removed.

The document referred to the Tops store as "attack area 1" and described two other nearby locations as attack areas to "shoot all blacks," the Post reported.

The writer said he counted 53 Black people and six white people in the Tops at the time of his visit, according to the account.

The document also mentioned that the author was ticketed by police for speeding en route to Buffalo on March 8, and that a speeding citation issued to Gendron and obtained by the Post matched details of the online account, the newspaper said.

Police confirmed that they are investigating Gendron's online postings, which included a 180-page manifesto he was believed to have written outlining the "Great Replacement Theory," a racist conspiracy theory that white people were being replaced by minorities in the United States and elsewhere.

Experts say the trend of mostly young white men being inspired by previous racist gun massacres is on the rise, citing recent mass shootings, including the 2015 attack at a Black church in Charleston, South Carolina, a 2018 shooting at a synagogue in Pittsburgh and a 2019 attack at a Walmart in an Hispanic neighborhood of El Paso.

President Joe Biden and his wife Jill Biden plan to visit Buffalo on Tuesday.

US Representative Liz Cheney took to Twitter on Monday to call on fellow Republicans to reject white supremacy, saying the political rhetoric of her party’s leaders in the House of Representatives has "enabled white nationalism, white supremacy, and anti-Semitism."

At a separate news conference on Monday, civil rights attorney Ben Crump called on officials to define Saturday’s attack as an "act of domestic terrorism."

"We can’t sugarcoat it, we can’t try to explain it away talking about mental illness. No, this was an act of domestic terrorism perpetrated by a young white supremacist," Mr Crump said, surrounded by the weeping family of Ms Ruth Whitfield, an 86-year-old woman who was among those slain at the Tops supermarket.  

Other victims included a pharmacist, a church deacon, and a young man who pushed grocery carts and did other jobs.  

If the suspect had not been stopped, authorities said he planned to continue the killings, possibly targeting another large store nearby.

Gendron came to the attention of local law enforcement last June, when police detained him after he made a threat at his high school, Mr Gramaglia told reporters.

He was given a mental health evaluation and released after 1-1/2 days.  

Authorities said that Gendron on Saturday, dressed in body armour and a tactical helmet, arrived at the Tops store and began his assault with the semi-automatic rifle, which he had bought legally but then modified.

Law enforcement found an additional rifle and a shotgun in his car.

The gunman broadcast the attack in real time on the social media platform Twitch, a live video service owned by Amazon.com Inc. The video service said it removed the broadcast within minutes.