Attorneys for the gunman who murdered 10 Black people in a racially motivated mass shooting at a Buffalo grocery store are arguing the convicted killer should be exempt from the death penalty.

Since Payton Gendron was 18 years old at the time in May 2022 when he stormed Tops Supermarket armed with an AR-15 and sprayed unassuming shoppers in a predominantly Black neighborhood, his defense team said his brain was still undergoing cognitive development, inhibiting his decision making skills, and should thus be reason enough to spare his life.

"The science is ... clear and uniform: People under 21 are not yet adults and should not be punished as such," they wrote in their Monday filing, according to WHAM-TV. Gendron's lawyers argued against "executing individuals barely old enough to vote, unable to drink legally or rent a car, unable to serve in Congress, and still in the throes of cognitive development."

Gendron, who is white and now 20 years old, is currently serving life in prison on state charges of murder and hate-motivated domestic terrorism after he pleaded guilty to the slayings, according to the Associated Press.

But in a separate federal hate crime case, the government said it plans to pursue the death penalty, citing premeditation and a targeted location carefully chosen to "maximize the number of Black victims," the AP reported.

"Payton Gendron expressed bias, hatred, and contempt toward Black persons and his animus toward Black persons played a role," court documents filed earlier this year read, according to ABC News.

Gendron drove more than 200 miles from his Conklin, New York, to carry out the massacre that claimed the lives of victims between the ages of 32 and 86.