Police in Grand Forks, North Dakota, don't know what prompted a U.S. airman to walk into a Wal-Mart Supercenter and kill one worker, injure another and then fatally shoot himself.

Lt. Derik Zimmel says "there are likely more questions than answers" as to why 21-year-old Marcell Willis walked into the store shortly after 1 a.m. Tuesday and opened fire.

Zimmel says the shooting appears to have been random. They haven't found a link between Willis and the Wal-Mart store or the employees.

Willis was stationed at Grand Forks Air Force Base, about a dozen miles west of the city.

Authorities did not immediately identify the two workers who were shot. The injured person was taken to a Grand Forks hospital.