Buffalo shooting: Red flags were raised about suspected gunman before killing spree

Questions are being asked about red flags raised over a teenage gunman who went on a racist killing spree at a US supermarket.

Police say Payton Gendron, 18, carried out an act of "racially motivated violent extremism" when he live-streamed himself opening fire with a semi-automatic rifle on Saturday at a supermarket in Buffalo, New York.

Ten people were killed and three injured. Eleven of those were black.

And now, troubling details of the teenager's behaviour in high school and his online presence, have begun to emerge.

Buffalo Police Commissioner Joseph Gramaglia said Gendron appeared on the radar of local officers last June, when they detained him after he made a "generalised" threat at his school.

The teenager was given a mental health evaluation at the time but was released a little over a day later.

Then, a 180-page "manifesto" circulated online, which was believed to have been written by Gendron, outlining the "Great Replacement Theory", a racist conspiracy that claims white people are being replaced by minorities in the United States and elsewhere.

Another online document appeared to also have been written by Gendron which sketched out a to-do list for the attack, including cleaning the gun and testing the livestream he would use to relay it on social media.

An emotionally-charged news conference was given on Monday by the family and friends of Ruth Whitfield, one of the victims, who was described as a "hero" for her community and an "angel" for her family.

Their lawyer Benjamin Crump was forced to pause his address as two of the women collapsed, sobbing.

One had to be helped to a nearby seat and comforted.

Not just the gunman who loaded the weapon

Mr Crump said: "What happened on Saturday was an act of domestic terrorism and we have to define it as such - we can't sugar-coat it.

"Just like American responds to terrorism, American needs to respond to this act of bigotry, racism and hate, as a terrorist act."

He went on: "We intend to not only hold accountable the sick, depraved monster responsible for this hateful act, but also those responsible at the root, those people who curate the hate, and the people who inspire the hate on websites, internet services, cable news stations - the people who radicalise young people.

"They did load the gun - and we need to hold them accountable too."

Gendron surrendered to police after the shooting and was charged with first-degree murder, which carries a maximum term in New York of life in prison without parole, but he has pleaded not guilty.