The mine appeared to be from the World War II era but was not the type used by the American Navy during that period, according to the curator for the U.S. Naval Undersea Museum in Keyport.

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(This story has been updated here.)

The U.S. Navy says it uses naval mines in Puget Sound for training purposes but still hasn’t determined the origins of an old, inert mine found floating off Bainbridge Island on Tuesday.

The mine — covered by decades of growth — broke loose from an anchor, said Sheila Murray, the Navy’s Region Northwest spokeswoman. Murray said she could not speculate on the origins of the mine until the Navy finishes its investigation.

The mine, which the Navy said “appears to be a decades-old military mine of unknown origin,” was detonated Tuesday night.

An explosive device was drifting Tuesday afternoon near the Port Brownsville Marina in Kitsap County. The Coast Guard moved it to deeper waters and detonated it shortly after 8 p.m. (Mike Griffith/KIRO 7)

Naval historian and military technology analyst Norman Friedman, who worked for a decade as an adviser to secretaries of the Navy, said that based on photos, it looks like a spherical mine, which was commonly used for training.

“The odds are, it’s a practice mine,” Friedman said. “The fact that it’s lying around suggests someone laid it for an exercise and never recovered it.”

There isn’t any evidence that anyone was laying mines along the West Coast during the war, Friedman said. The U.S. Navy may have had a defensive minefield in Puget Sound to protect the shipyard, but Friedman said this is unconfirmed and would have been dangerous for merchant ships.

Mary Ryan, curator of the U.S. Naval Undersea Museum in Keyport, said personnel from the Navy’s Explosives Ordnance Disposal told her the mine appeared to be a pill-shaped contact mine from the World War II era.

If this is the case, these types of mines were not used by the American Navy during that period, Ryan said, as American mines were more spherical. However, she acknowledged that information was not definitive yet. She said it was not known whether the mine was foreign in origin.

Murray said she did not have information about the shape of the mine Wednesday afternoon.

The mine was spotted Tuesday afternoon floating about 400 yards east of Brownsville Marina in Kitsap County. Brownsville is a few miles south of Naval Base Kitsap.

Boat traffic in and out of the Brownsville Marina was stopped and the Kitsap County Sheriff’s Office urged people who live along the shoreline between Illahee Dock and Keyport to shelter in place and stay off the beaches.

The mine appeared to be a large, round, rusted object, maybe 5 feet in diameter, with rods protruding from it.

The Navy said Tuesday that it could not immediately determine whether the mine was inert and did not believe it could be safely towed to shore for further examination. Navy demolition experts used explosives late in the day to detonate the mine.

The primary explosion did not trigger a second, larger blast, which indicated that the mine was inert, Navy officials said.

Sheriff’s Deputy Scott Wilson said he doesn’t remember anything like this happening in his 24 years with the agency.