A man buying treats for his young son's birthday. A woman just back from visiting her husband at a nursing home. A security guard who knew every shopper by name. These are just three of the victims of a deadly mass shooting in the city of BuffaloThe second-largest city in the state of New York. It sits on the Canadian border., New York.

On Saturday, 18-year-old Payton Gendron drove 320km from his hometown of Conklin, NY, seeking an area with a large Black population. He parked outside a supermarket, switched on a live streamGendron broadcast the shooting on the gaming platform Twitch, allowing others to watch from his own perspective as he murdered innocents. and marched through the car park and into the shop, shooting as he went.

By the time the police persuaded him to surrender, he had shot 13 people. Ten of them died.

In the wake of the attack, Americans are devastated.

Some think the US security services are to blame. Last year Gendron was investigated by state police and subjected to a psychiatric evaluation. He was released after a couple of days and neither the police nor counter-terrorism forces kept him under any surveillance.

Others blame lax gun laws. Gendron was carrying a rifle and wearing body armour. Security guard Aaron Salter, one of the victims, shot at him several times but could not penetrate his bulletproof vest. They argue it should be impossible for a mentally unstable 18-year-old to buy full military gear and use it to unleash terror on unarmed civilians.

But for many, there is one main culprit: the White supremacist conspiracy theory known as the Great ReplacementA racist conspiracy theory devised by French author Renaud Camus. It is also known as "white genocide". In Europe, it is particularly associated with Islamophobia. The theory also has anti-Semitic undertones: its advocates often allege that Jewish groups are responsible for orchestrating the replacement., which Gendron cited in his 180-page manifestoA statement published by a person or group of people, especially a political party, or a government, in which they say what their aims and policies are..

According to this theory, there is an international plot to eliminate White people. Rather than killing them, the theory goes, the perpetrators of the Great Replacement are instead "replacing" White people in western societies with people of colour, through migration and a drop in White birth ratesThe rate at which babies are born, usually expressed as a number per 1,000 people. The global birth rate is currently falling..

For years, replacement theory was limited to a White supremacist fringe. But it has recently gained more mainstream acceptance thanks to conservative media. 

A recent poll found almost half of Republicans believe a group of people is trying to replace "native-born" Americans with migrants. The survey also found they were far more likely to hold such views if they watched right-wing cable news like One America News, Newsmax and America's biggest cable news network: Fox NewsAmerica's biggest cable news channel, with an average total prime time audience of 2.6 million viewers. It is generally believed to have a strong right-wing bias..

One of the biggest draws on Fox News is Tucker CarlsonAn American conservative broadcaster and TV show personality. He is best known for his former show on Fox News. , presenter of the biggest cable news programme in history. Carlson has become a champion of replacement theory, telling his 4.33 million nightly viewers that the Democratic Party is encouraging the growth of the non-White population because it sees them as guaranteed votes.

Gendron says he was radicalised by online imageboardsA kind of internet forum revolving around the posting of images alongside text and discussion. Some, including 4Chan and its spin-off 8Chan, have become hubs of far-right propaganda., not cable news. Indeed, he has suggested that Fox News itself supports the replacement of White People. But some think Tucker Carlson - and Fox - are nonetheless responsible for bringing the theory into the mainstream. 

For them, this is not the first attack motivated by similar rhetoric - and will not be the last.

Yes: Tucker Carlson's critics say he has done more than anyone else to bring the Great Replacement into the mainstream. Millions of people now believe the same racist lies as the Buffalo shooter.

No: Most White nationalist terrorists are still radicalised online, not by cable news. We need to be focusing much more on anonymous messaging boards, and not on cynical journalists pushing racism for ratings.

Or... Fox News is probably not radicalising racist terrorists. But it might well be making an ever larger proportion of the US population sympathise with them. That should worry us just as much.