The weapon used in the mass-shooting in Buffalo was obtained legally but was modified later with “illegal magazines” available on sale in other states like Pennsylvania, according to New York Governor Kathy Hochul.
The magazines used in the attack are not allowed to be sold in New York, officials said.
An 18-year-old white male from New York opened fire at a Tops Friendly supermarket in Buffalo on Sunday shot at least 13 people, out of which 11 were Black, Buffalo police commissioner Joseph Gramaglia said.
Calling the gunman “a white supremacist”, Ms Hochul said he terrorised New York’s second-largest city in a "cold-hearted," "military-style execution" while people were buying groceries.
"It strikes us in our very hearts to know that there’s such evil that lurks out there,” she said.
“This individual – this white supremacist – who just perpetrated a hate crime on an innocent community, will spend the rest of his days behind bars. And heaven help him in the next world as well," the governor said.
“Tonight my heart is with my neighbors in Buffalo who are feeling unimaginable pain due to a white supremacist. We’ll be there for the families affected by this act of terrorism,” she wrote on Twitter.
She added: “And we’ll ensure law enforcement has what they need to prosecute the shooter to the fullest extent.”
The suspect — Payton Gendron from New York’s Conklin — has been charged with murder and was presented before a judge in paper white gown shortly after he went on a shooting spree.
A video clip shot by the suspected gunman showed Payton entering the supermarket in a car.
Right outside the market premises, he shot down four people - out of which three were fatally shot - and he went inside after which multiple shots were heard, police commissioner Gramaglia said.
Several shots were fired by the market’s security guard Aaron Salter, a former police officer who has retired from the Buffalo police force, the commissioner said.
However, the bullet hit Payton’s bulletproof vest and had no effect.
The gunman then shot dead the guard and went inside the store — killing more victims.
Minutes later, police entered the store and confronted Payton but he put the rifle to his own neck.
Two officers managed to convince him into surrendering and dropping the gun, the commissioner said.