A teenage gunman entranced by a white supremacist ideology known as replacement theory opened fire at a supermarket in Buffalo on Saturday, methodically shooting and killing 10 people and injuring three more, almost all of them Black, in one of the deadliest racist massacres in recent American history.

The authorities identified the gunman as 18-year-old Payton S. Gendron of Conklin, a small town in New York’s rural Southern Tier. Gendron drove more than 200 miles to mount his attack, which he also livestreamed, the police said, a chilling video feed that appeared designed to promote his sinister agenda.

Shortly after Gendron was captured, a manifesto believed to have been posted online by the gunman emerged, riddled with racist, anti-immigrant views that claimed white Americans were at risk of being replaced by people of colour. In the video that appeared to have been captured by the camera affixed to his helmet, an anti-Black racial slur can be seen on the barrel of his weapon.

The attack, at a Tops Friendly Market in a largely Black neighbourhood in east Buffalo, conjured grim comparisons to a series of other massacres motivated by racism, including the killing of nine Black parishioners at a church in Charleston, SC, in 2015; an anti-semitic rampage in a Pittsburgh synagogue in 2018 that left 11 people dead; and an attack at a Walmart in El Paso in 2019, where the man charged had expressed hatred of Latinos. More than 20 people died there.

In the Buffalo grocery store, where four employees were shot, the savagery and planning were evident: Gendron was armed with an assault weapon and wore body armour, the police said. And his preferred victims seemed clear as well: 11 of the people shot were Black and two were white, the authorities said.

“It was a straight up racially motivated hate crime,” John Garcia, the Erie County sheriff, said.

In a news conference on Saturday evening, governor Kathy Hochul — a Buffalo native — decried the attack as an “act of barbarism” and an “execution of innocent human beings”, as well as a frightening reminder of the dangers of “white supremacist terrorism”.

In the manifesto, which was being reviewed by law enforcement, Gendron — who had attended a community college in Binghamton, New York— wrote that he had selected the area because it held the largest percentage of Black residents near his home in the state’s Southern Tier, a predominantly white region that borders Pennsylvania.

The document outlined a careful plan to kill as many Black people as possible, complete with the type of gun he would use, a timeline, and where he would eat beforehand. It also included details of where he would livestream the violence, mayhem that he had also calibrated. He carefully studied the layout of the grocery, writing that he would shoot a security guard before stalking through aisles and firing upon Black shoppers. He wrote that he would shoot some twice, in the chest, when he could.

He wrote he had been “passively preparing” for the Buffalo attack for several years, purchasing ammunition and gear, while infrequently practising shooting. In January, the plans “actually got serious,” according to the manifesto, which also praised the perpetrator of the 2015 attack in South Carolina, and a man who killed 51 Muslims at two mosques in Christchurch, New Zealand, in 2019.

In an arraignment on Saturday evening, Gendron pleaded not guilty to first-degree murder, a charge that could lead to life imprisonment without parole. He spoke little except to confirm he understood the charges, and gave little indication of emotion inside the courtroom.

The US attorney in Buffalo, Trini E. Ross, said her office was also investigating the killings as federal hate crimes.

Other gunmen have referenced the racist idea known as “replacement theory”, a concept once associated with the far-Right fringe, but one that has become increasingly mainstream, pushed by politicians and popular television programmes.

The massacre began around 2.30pm, the authorities said, when Gendron arrived at the market stepping out of his car — on a sunny spring afternoon — dressed in tactical gear and body armour and carrying an assault weapon.

He shot four people in the parking lot, the Buffalo police commissioner, Joseph A. Gramaglia, said at the news conference, three of them fatally. When he entered the store and continued shooting, he encountered a security guard, a retired Buffalo police officer who returned fire. But Gendron was wearing heavy metal plating; he killed the guard and continued into the store, firing on shoppers and employees.

When Buffalo police officers arrived and confronted Gendron, he put a gun to his neck, but two patrolmen persuaded him to drop his weapon and surrender, Gramaglia said.

President Joe Biden expressed sympathy for the victims’ families and praise for law enforcement, adding that “a racially motivated hate crime is abhorrent to the very fabric of this nation”.