Deputies were called to Viejas Casino in Alpine Friday to investigate a suspicious device discovered by a casino employee in a trash can inside a restroom.
The San Diego Sheriff's Department said concerned casino employees called deputies around 1:15 p.m. to report that they had discovered what appeared to be some type of grenade.
When authorities arrived at the scene in the 5000 block of Willows Road, they secured the area and a Sheriff's Bomb Arson Unit began investigating.
After several hours, officials determined the device lin the trash can was a military smoke grenade.
Investigators do not know who left the smoke grenade behind or why it was left there.
The San Diego County Sheriff's Department said there were no threats made toward the casino and no one was injured.
The incident remains under investigation.