BOULDER, Colo.—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, …