Blaine County Sheriff Gene Ramsey told the Idaho Mountain Express newspaper that the device he describes as "very crude" didn't operate as intended.
He says he doesn't believe it was intended to hurt anyone, but had it gone off effectively with someone nearby it could have caused injury.
The bomb was found April 24 in the mailbox by a caretaker at the residence between Hailey and Ketchum.
Ramsey describes the device as a "chemical pressure device" that was intended to explode when pressure inside reached a certain level. He also says the device contained metal objects he describes as shrapnel.
No suspects have been named.
Information from: Idaho Mountain Express, http://www.mtexpress.com
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