The Ada County Sheriff's Office says that the device was located just to the north of Shadow Valley Golf Course.
It was spotted on a turnout on the east side of the road.
Police say the device appeared credible enough to call in the bomb squad and dogs to investigate.
Officials now say the device is not a bomb, but they are investigating the area as a crime scene.
Police say there is a message written on the ground in the area where the device was found that they are analyzing.
The message includes the expletive phrase "F*** U LA FBI" and "Free us Obama" and has a drawing of an American flag.
The highway re-opened shortly after 6 p.m.