The Coast Guard and Navy bomb technicians detonated what they said appeared to be an unidentified mine floating in Puget Sound between Brownsville Marina and Bainbridge Island.
A Coast Guard spokeswoman said the object was reported at about 2 p.m. Tuesday.
The Navy says initial inspection of the moored mine showed it had decades of marine growth. At about 5 p.m., Navy divers secured a long line to the device and began towing it with a small boat. By 8:15 p.m., the device was detonated without incident.
The Navy said the mine did not cause a second explosion when detonated, which means it was not active.
Authorities had asked residents along the waterfront to stay inside and away from beaches as a precaution.
Chopper 7 flying overhead showed the object was a round, rusted object with rods protruding from it.
Brownsville is a few miles south of Naval Base Kitsap — Keyport's torpedo testing range.
On Thursday, the Navy said the mine came from an undersea warfare exercise conducted in 2005.
During the exercise, inactive training mines were placed in areas between Brownsville, Keyport and Bainbridge Island, and not all of them were recovered.
The Navy says it's going to survey the exercise areas and recover any other training mines to ensure a another incident doesn't occur again.
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