BOULDER, Colo. (AP) — The suspect in the Colorado supermarket shootings bought a firearm at a local gun store after passing a background check, and he also had a second weapon with him that he didn’t use in the attack that killed 10 people this week, authorities and the gun store owner said Friday.

Investigators are working to determine the motive for the shooting, but they don’t know yet why the suspect chose the King Soopers grocery store in Boulder or what led him to carry out the rampage, Police Chief Maris Herold said at a news conference.

“Like the rest of the community, we too want to know why — why that King Soopers, why Boulder, why Monday,” Herold said. “It will be something haunting for all of us until we figure that out. Sometimes you just don’t figure these things out. But, I am hoping that we will.”

The quick response by officers, who traded gunfire with the suspect, kept many people inside the store out of danger, said Boulder County District Attorney Michael Dougherty, who declined to say how many people were in the supermarket. The first officer on scene was killed.

“Their actions saved other civilians from being killed,” Dougherty said about the officers. “They charged into the store and immediately faced a very significant amount of gunfire from the shooter, who at first they were unable to locate.”

More charges will be filed against 21-year-old Ahmad Al Aliwi Alissa in the coming weeks in connection with firing at officers, Dougherty said.

John Mark Eagleton, owner of Eagles Nest Armory in the Denver suburb of Arvada, said in a statement that his store was cooperating with authorities as they investigate. The suspect passed a background check conducted by the Colorado Bureau of Investigation before purchasing a gun, Eagleton said.

Alissa used a Ruger AR-556 pistol, which resembles an AR-15 rifle with a slightly shorter stock, in the shooting, Herold said. An arrest affidavit says Alissa purchased it on March 16, six days before the shooting.

He also had a 9 mm handgun, which the police chief said was not believed to have been used in the attack. Herold didn’t say how Alissa obtained it.

“We are absolutely shocked by what happened and our hearts are broken for the victims and families that are left behind. Ensuring every sale that occurs at our shop is lawful, has always been and will always remain the highest priority for our business,” Eagleton said in the statement.

The gun store is in a shopping center that also has a chiropractic clinic, yoga studio and foot massage parlor. It is less than a half-mile from one of the restaurants Alissa’s family owns and about 3 miles (5 kilometers) from his family’s house in Arvada. A different King Soopers store is across the street.