The vessel was lost without a trace in 1921.

Government researchers say they’ve found a US Navy tugboat that went missing nearly a century ago.

The 170ft USS Conestoga left San Francisco on 25 March, 1921, bound for Pearl Harbor in the Hawaiian islands, and carrying 56 crew onboard.

When the ship failed to arrive at its destination two weeks later, the largest air and sea search effort that had ever been mounted began.

Dozens of Navy destroyers swept hundreds of thousands of square miles of the Pacific Ocean to no avail, finding no trace of the vessel. The USS Conestoga was eventually declared lost with all hands in June 1921.

Officials from the National Oceanic and Atmospheric Administration first detected the ship’s remains while mapping the seafloor in waters off the California coast in the Greater Farallones National Marine Sanctuary.

NOAA marked the site as a probable shipwreck, and a NOAA maritime heritage researcher, Robert Schwemmer, began to investigate the unknown object in 2012.

Schwemmer took an interest in the object and directed a two-year investigation, comparing it to old photographs and news clippings of shipwrecks in the region.

When divers visited the site in late 2014, Schwemmer matched the underwater images to the USS Conestoga.

The ship’s identity was confirmed in October 2015.

A press release from NOAA in March 2016 said the boat’s discovery solves one of the “top maritime mysteries in U.S. Navy history.”

“After nearly a century of ambiguity and a profound sense of loss, the Conestoga’s disappearance no longer is a mystery,” said Manson Brown, assistant secretary of commerce for environmental observation and prediction and deputy NOAA administrator.

“We hope that this discovery brings the families of its lost crew some measure of closure and we look forward to working with the Navy to protect this historic shipwreck and honor the crew who paid the ultimate price for their service to the country.”

The exact events leading up to the sinking of the WWI-era will likely remain a mystery, however, as the crew never sent a distress call.

According to NOAA, weather logs indicate that around the time of Conestoga’s departure, the wind in the Golden Gate area increased from 23 miles per hour to 40 miles per hour, and the seas were rough with high waves. A garbled radio transmission from Conestoga relayed later by another ship stated the tug was “battling a storm and that the barge she was towing had been torn adrift by heavy seas.”

Based on the location and orientation of the wreck in 189-foot-deep water, three miles off Southeast Farallon Island, NOAA, and its technical and subject matter experts, believe Conestoga sank as officers and crew attempted to reach a protected cove on the island.

Originally built to tow coal barges for the railroad, the Navy purchased Conestoga in 1917 for World War I service. The tug operated on the Atlantic coast and off the Azores, performing convoy and other duties before being assigned to harbor service in Norfolk, Virginia, in 1919. Ordered to duty in American Samoa, Conestoga steamed from Mare Island Naval Shipyard in California at 3:25 p.m. on March 25, 1921, headed for Pearl Harbor. After leaving the Golden Gate, the tug, possibly towing a barge, was never heard from again.

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