SPOKANE, Wash. -

The Spokane County Sheriff's Office is investigating an armed robbery at a north Spokane McDonald's on Monroe and Francis Tuesday night.

Investigators say the suspect approached the drive-thru window on foot while the cashier was with a customer. He displayed a black revolver, then reached in and grabbed the money tray out of the open register.

The suspect was last seen headed eastbound on foot, but a K9 track found nothing.

The suspect is described as a white male wearing a grey hoodie with a black piece of cloth covering his face.