President Biden called for a thorough investigation, and said there was no harbor for “hate-filled domestic terrorism.” The 18-year-old white gunman, who pleaded not guilty, left behind a manifesto.

BUFFALO — 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. Mr. 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 Mr. 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 color. 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 neighborhood 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, S.C., in 2015; an antisemitic 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: Mr. Gendron was armed with an assault weapon and wore body armor, the police said. And his preferred victims seemed clear as well: All told, 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 Saturday evening, Gov. Kathy Hochul — a Buffalo native — echoed that sentiment and 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.”

“It strikes us in our very hearts to know that there is such evil that lurks out there,” Governor Hochul said.

Based on what was written in the manifesto, the attack appeared to have been inspired by earlier massacres that were motivated by racial hatred, including a mosque shooting in New Zealand and the Walmart shooting in Texas, both in 2019.

In the manifesto, which was being reviewed by law enforcement, Mr. Gendron — who had attended a community college in Binghamton, N.Y. — 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 predominately 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 practicing shooting. In January, the plans “actually got serious,” according to the manifesto, which also expressed praise for the perpetrator of the 2015 attack in South Carolina, and for a man who killed 51 Muslims at two mosques in Christchurch, New Zealand, in 2019.

Mr. Gendron had read the racist writings of the New Zealand gunman, who had also livestreamed his attack, a method also used in a shooting at a Jewish synagogue in Halle, Germany, in 2019.

In an arraignment on Saturday evening, Mr. 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 United States 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 programs.

Officials said the camera that the gunman wore was used to broadcast the attack live on Twitch, a livestreaming site owned by Amazon that is popular with gamers. On Saturday, Twitch said it had taken the channel offline. Still, screenshots of the broadcast were circulating online, including some that appeared to show the shooter holding a gun and standing over a body in the grocery store.

In his manifesto, Mr. Gendron seemed enthusiastic about broadcasting his attack, saying the livestream let “all people with the internet” watch and record the violence.

The massacre began around 2:30 p.m., the authorities said, when Mr. Gendron arrived at the market stepping out of his car — on a sunny spring afternoon — dressed in tactical gear and body armor 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 Mr. 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 Mr. Gendron, he put a gun to his neck, but two patrolmen persuaded him to drop his weapon and surrender, Mr. Gramaglia said.

The mayor of Buffalo, Byron W. Brown, said that he and his family periodically shopped at the store.

“Some of the victims of this shooter’s attack are people that all of us standing up here know,” said Mr. Brown, the fifth-term Democrat who was the first Black man elected mayor of Buffalo, New York’s second-most populous city.

The 10 people killed in Buffalo represent the highest number of fatalities in a mass shooting in the United States this year, according to the Gun Violence Archive, which tracks them. The highest death toll this year before that was six, in a shooting in downtown Sacramento on April 3. Six people were also killed in a shooting in Corsicana, Texas, on Feb. 5, and the same number were killed in a shooting in Milwaukee on Jan. 23, according to the site.

In a statement made late Saturday night, President 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.”

“Any act of domestic terrorism, including an act perpetrated in the name of a repugnant white nationalist ideology, is antithetical to everything we stand for in America,” the president said. “Hate must have no safe harbor.”

Gun deaths reached the highest number ever recorded in the United States in 2020, the first year of the pandemic, surging by 35 percent, the Centers for Disease Control and Prevention reported on Tuesday.

The gunfire in Buffalo on Saturday shattered a seemingly serene afternoon, sending shoppers screaming and fleeing inside the Tops, and families scrambling to find loved ones outside the store.

Ken Stephens, 68, a member of a local anti-violence group, described a grisly scene. “I came up here, and bodies were everywhere,” he said.

The attack took place in a neighborhood known as Masten Park on Buffalo’s East Side. Dominique Calhoun, who lives within sight of the supermarket, said she was pulling into its parking lot to buy ice cream with her daughters — eight and nine years old — when she saw people running out and screaming.

“That literally could have been me,” she said of the people who were killed.

Dorothy Simmons, 64, typically spends part of her Saturdays at Tops, shopping for food to prepare for Sunday dinner, something she says is part of a common tradition in her community. On Saturday, however, she was at work in Amherst.

And when she heard the news, she broke down and cried.

“This is our store,” Ms. Simmons said. “This is our store.”/agencies