A mass shooter opened fire at an Idaho In-N-Out Burger, leaving at least three people dead and seven injured, according to authorities. 

The mayhem began Saturday afternoon at the burger joint in Twin Falls -- a city in southern Idaho -- where the gunman opened fire, officials said.

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Twin Falls spokesperson, Josh Palmer, told TMZ ... police received a call of an active shooter and quickly responded, but the shooter had already exited the restaurant and fled the immediate area.

A motorist filmed the shooter with a cell phone camera walking to a white car while carrying a long gun. The suspect opened the driver-side door and popped the trunk, then walked to the rear of the car and reached inside the trunk. Fearing the suspect might be reaching for another firearm, the motorist took off.

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Palmer said the cops briefly searched for the gunman, finding him dead nearby. It's unclear how he died or if he was counted in the initial death toll. Palmer said at least 3 people were dead, and seven others were injured. 

Police were interviewing numerous witnesses and canvassing for video at retail shops near the crime scene. Investigators were also trying to identify the suspect and determine a motive. Palmer said there were no other shooters and the threat to the public was over.