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Watch: US President Joe Biden says "we must work together to address the hate that remains a stain on the soul of America"
A gunman who killed 10 people at a supermarket in Buffalo, New York, planned further attacks after the mass shooting on Saturday, police have said.
The suspect, 18, drove more than 320km (200 miles) to carry out what is believed to be a racially-motivated crime.
He planned to keep driving and "shoot more black people," Buffalo's police chief told US media.
A pensioner, a former policeman and a grandmother were among the victims.
Mayor Byron Brown said the suspect arrived in Buffalo intending to take "as many black lives as possible".
A 180-page document seemingly authored by the alleged attacker Payton Gendron has emerged, in which he describes himself as a fascist and a white supremacist.
Questions are being asked about how he was able to carry out the shooting when concerns had already been raised. "I want to know what people knew and when they knew it," New York Governor Kathy Hochul told ABC News.
FBI officials have confirmed that the gunman spent a day and a half in hospital undergoing a mental health evaluation last year after he wrote that he wanted to commit a murder/suicide in a high school project.
No criminal charges resulted from the incident, and Buffalo police chief Joseph Gramaglia told reporters the gunman had only made "generalised threats".
He does not appear to have remained under watch by authorities.
Mr Gramaglia told US media that there was evidence to suggest he wanted to targeted a second store on Saturday if he had not been stopped.
On Monday, police said he had visited Buffalo in March and then again one day before the deadly mass shooting
Meanwhile, the gun store owner who sold the semi-automatic used in the attack told several US outlets that no alert came up when he ran the suspect's name through a government background check system.
New York's Attorney General Letitia James said her office would focus on extremist material online.
"This event was committed by a sick, demented individual who was fuelled [by a] daily diet of hate," she said.
The shooting has stunned the local community. One of those attending a vigil on Sunday told Reuters: "It just hurts, why somebody would do that."
Ten died and three others were injured.
Of the 13 people shot, 11 were black. Among those reported killed were a man buying cupcakes for his son's birthday and a woman who had gone shopping after visiting her husband at a nursing home.
By Mike Wendling, BBC Trending
Christchurch, El Paso, Pittsburgh and now Buffalo – all places where racially-motivated assailants, radicalised online, have taken their ideology to deadly extremes.
The gunman in Buffalo, like ones before, live-streamed his violent rampage and left a so-called "manifesto" online. It details his extremist beliefs and is packed with cherry-picked statistics, conspiracy theories and internet memes.
The file contains reams of racist and anti-Semitic sludge along with straightforward admissions that the author is a fascist and a white supremacist.
If the author can be believed – as the document also clearly contains disinformation and attempts to trick reporters into reporting false stories – he was radicalised early on during the Covid pandemic, on extremist websites and message boards.
Like after the 2019 mosque shootings in Christchurch, big social media companies will struggle to remove footage of the attack.
And surely the American debate over gun control will be reignited, however briefly.
But the underlying problem seems as intractable as ever: a worldwide network of young violent extremists, some of whom are motivated to launch deadly attacks against innocent people.
The attacker, dressed in military gear, drove into the car park at Tops Friendly Market in Buffalo – an area in which police said he had done "reconnaissance" the day before – at about 14:30 EST (19:30 BST) and began livestreaming the rampage.
A security guard fired several shots back but the gunman's bulletproof vest stopped one that hit him, police said. He then killed the guard and stalked through the store firing at other people.
Witnesses described horrific scenes. "It's like a nightmare… you see this on TV, you hear about it on TV… but I never thought I would be one of them," said one.
Among the victims was Katherine Massey, 72, who had written in favour of federal gun legislation for her local paper less than a year ago.
The gunman was arrested after the attack and has pleaded not guilty to murder charges.
US President Joe Biden said facts were still being established but strongly condemned racist extremism. "We must all work together to address the hate that remains a stain on the soul of America," he said.
The White House said Mr Biden and the First Lady would travel to Buffalo on Tuesday to meet with the community.
Saturday's attack is thought to be the worst mass shooting so far in the US in 2022. Some 40,000 deaths a year involve firearms in America, a figure that includes suicides – and mass shooting events occur frequently.
Less than a day after the attack in New York state, one person was killed and five were injured in a shooting at a church in Orange County, in southern California.
The gunman was a Chinese immigrant apparently motivated by hatred for Taiwanese people, authorities said on Monday.
Churchgoers tackled him and tied him up by the ankles using an extension cord.
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