Police say a U.S. airman was acting alone when he fatally shot one worker and injured another at a North Dakota Wal-Mart store before killing himself, and that no one had any advance notice of his intentions.

Grand Forks police said Wednesday that they closed their investigation into the May shooting without forwarding the case to prosecutors for possible charges.

Twenty-one-year-old Marcell Willis was a senior airman stationed at Grand Forks Air Force Base. Police say he walked into the Wal-Mart store about 1 a.m. on May 26 and shot and killed 70-year-old overnight cashier Gregory Weiland. Willis then shot and wounded 47-year-old overnight grocery general manager Lisa Braun.

Willis then killed himself. His motive isn't known.