A store where the suspect in the Colorado supermarket massacre bought his gun reportedly said the sale was lawful after he passed a background check.

Ahmad Al Aliwi Alissa, 21, bought his AR-15-style Ruger AR-556 at the Eagles Nest Armory in Arvada, about 20 miles southeast of Boulder, 9NEWS reported.

The store’s owner, John Mark Eagleton, said the accused gunman passed a background check before making the purchase.

“We are absolutely shocked by what happened and our hearts are broken for the victims and families that are left behind,” he wrote in a statement to the outlet.

“Ensuring every sale that occurs at our shop is lawful, has always been and will always remain the highest priority for our business,” Eagleton continued.

“Regarding the firearm in question, a background check of the purchaser was conducted as required by Colorado law and approval for the sale was provided by the Colorado Bureau of Investigation,” he wrote.

“We have and will continue to fully cooperate with law enforcement as their investigation continues,” the owner added.

Alissa bought the Ruger on March 16, shortly after the firearm was made legal in Boulder, where he allegedly slaughtered 10 people at a King Soopers supermarket six days later.

The Syrian native — who faces 10 counts of first-degree murder and one count of attempted murder — appeared in court Thursday for a procedural hearing in the mass murder case.

He remains in the custody of the Boulder County Sheriff’s Office, but is being housed in a correctional facility outside the county after jail staff learned of safety concerns and threats.