By Agueda Pacheco-Flores and Hal Bernton / The Seattle Times

Navy demolition experts used explosives late Tuesday to attempt to detonate an old military mine found mysteriously floating near Brownsville Marina in Kitsap County.

News video, shot in waning light, showed a fountain of water erupting from a blast as a number of Coast Guard and military water craft are seen in the background.

The Navy said the primary explosion did not trigger a second, larger blast, indicating that the mine was inert. Navy officials said its “origins remain undetermined” and that an investigation is ongoing.

Earlier Tuesday, Navy, Coast Guard and law-enforcement officials scrambled to respond to what appeared to be an unexploded mine, covered by decades of growth, that was seen mysteriously floating near Brownsville Marina in Kitsap County.

The Navy said that responding Navy explosive-ordnance officials “have determined through an abundance of caution the safest public measure to take is to detonate the object found in the waterway earlier today, which appears to be a decades-old military mine of unknown origin.”

The Navy said it could not determine if the mine was inert and did not believe it could be safely towed to shore for further examination. The Navy said it towed the device to an undisclosed location where it would be rigged to detonate.

The Navy said that people in the area should expect “an audible explosion before sunset” Tuesday, but said no danger to the public was expected. Out of caution, people were asked to stay away from shorelines and shelter in place.

The U.S. Coast Guard via Twitter said that the device was neutralized at 8:04 p.m. and officials canceled a security zone and lifted flight restrictions that had been put in place.

The Coast Guard originally placed a 1,500-yard safety zone around the mine after it was spotted by a state Department of Natural Resources crew drifting 400 yards east of the Brownsville Marina. A 5,000-foot flight-restriction zone was also temporarily established. The device appears to be a naval mine, although the Navy has not confirmed the nature or the source of the device.

The statement from the Navy said the device appears to be an “unidentified mine” that was moored at some point and shows “decades of marine growth.”

The Kitsap County Sheriff’s Office told residents over Twitter to remain inside and stay away from the beaches.

At 3 p.m. Tuesday, boat traffic in and out of the Brownsville Marina was stopped, according to a woman who answered the phone at the marina. The Kitsap County Sheriff’s Office is asking those who live along the shoreline between Illahee Dock and Keyport to shelter in place and stay off the beaches.

Three men in a small inflatable boat approached the ordnance floating in calm waters at about 4:50 p.m., according to live stream from TV stations. Two of the men in wet suits jumped into the water, swam toward the device and used a rope to pull it from the area.

The device appeared to be a large, round, rusted object covered with rods protruding from it, maybe 5 or 6 feet in diameter based on its scale next to the two swimmers. It was first discovered by the state Department of Natural Resources, according to the Navy.

Upon initial inspection, the unidentified mine was moored, according to a statement released by a Navy spokeswoman.

Jack Bailey, president of the Port of Brownsville Commission, said he has received — via his phone — a shelter-in-place alert early Tuesday evening at his home near the marina. He said that the emergency notification said he should lock his doors and move away from windows.

Bailey noted that there was recently a low tide, with a good amount of current that can bring in floating objects. He also noted that the object appeared to have an anchor of some sort, that may have snagged on the bottom.

Bailey, who worked for the Navy as a civilian before retiring in 1996, said, “We have had other occasions when ordnances have been brought up. But never anything like this… I am amazed by the whole thing.”

One earlier incident, he recalled, involved a diver harvesting geoduck who retrieved a torpedo.

Brownsville is a few miles south of Naval Base Kitsap — Keyport’s torpedo testing range.

Information from The Associated Press is included in this story.