A suburban Detroit officer who fatally shot an armed man inside the lobby of police station will not face criminal charges, a prosecutor said Thursday.

Ali Naji "objectively posed an imminent threat" and the Dearborn police officer who shot him acted in self defense, Wayne County Prosecutor Kym Worthy said.

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Naji, 33, walked into the police station just west of Detroit on Dec. 18 and attempted to fire a handgun at the officer, who was seated at a desk behind a bullet-resistant window. The gun failed to fire. While Naji removed the magazine and attempted to reinsert it into the gun, the officer opened the window and shot Naji.

Naji, of Dearborn, was pronounced dead at a hospital. State police determined the gun he wielded had been stolen from a local barbershop where he previously had worked.

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Worthy's office said Naji had a known history of mental health issues and that on the day of the shooting people were in and out of the police station lobby dropping off gifts as part of an annual Christmas toy collection drive.

"We may never know why Mr. Naji walked into the Dearborn Police Department with a loaded weapon attempting to fire it at a police officer," Worthy said. "Although extremely tragic, this is a clear case where the officer acted in lawful self-defense and in the defense of others."