A homemade smoke bomb failed to ignite Monday in a bathroom at the Airway Heights Corrections Center, west of Spokane.
Associate Superintendent of Custody James Key says the device was a pop can with a small amount of mineral or baby oil in the bottom and a toilet-paper wick.
Key says the toilet paper had been lit, but went out.
The 300 inmates in the minimum security unit were evacuated when the device was found at 8:40 a.m. All are accounted for.
The Spokane city-county bomb squad secured the device. Key says it could have caused a lot of smoke if it had burned.
Airway Heights police and prison staff are investigating.
The prison, which holds 2,250 inmates, was back to normal by 12:30 p.m.