The rectangular, olive-green box with the words "Front Toward Enemy" raised the suspicions of Goodwill workers Tuesday.
Arvada police say the Claymore land mine didn't go off in the donation box and no one was hurt. A bomb squad disposed of the device.
Police Sgt. Jeff Monzingo says it's unclear whether the device was operational or where it came from. No suspects have been identified.
Claymore mines were widely used during the Vietnam war.
