A wreck-hunting group announced that it has located the sunken World War II submarine USS S-28 in 8,700 feet of water off Oahu.

The S-28, which was lost with 49 crew during training on July 4, 1944, is “considered to be one of the most important lost ships in the central Pacific,” STEP Ventures said on its website.

STEP, which stands for Search for Those on Eternal Patrol, is made up of historians and explorers with a mission of discovering and documenting submerged maritime history. The expedition used autonomous underwater vehicles and a remotely operated vehicle to map and film the wreck, and that data is being shared with the Navy to help determine the cause of the loss.

During the war, the S-28 was the only American submarine to go down in the main Hawaiian Islands area, said Charles Hinman, education director at the USS Bowfin Submarine Museum and Park on Oahu.

“We’re extremely confident that it’s the S-28 because it’s within the area that it was lost — very precisely,” said Tim Taylor, who led the expedition. The submerged vessel, which is in two sections, matches modifications made to the S-28, including a 20mm gun that was mounted in the aft tower, he said.

The S-28 was commissioned on Dec. 13, 1923. “It was a riveted hull,” Taylor said in a phone interview. “This is a World War I-era vessel that was brought back for World War II,” he said.

On Dec. 7, 1941, the 219-foot diesel sub was being overhauled near San Francisco. It was initially sent to Alaska to defend the Aleutians and forayed periodically to the Kuril Islands, but recorded just one ship sinking in 1943.

The Naval History and Heritage Command said on its website that the sub got underway from Pearl Harbor on July 3, 1944. The vessel made two practice torpedo approaches on the Coast Guard cutter Reliance.

A day later about 15 miles off the Waianae Coast the S-28 again carried out sonar exercises and made another practice approach on Reliance.

At 5:30 p.m. S-28 dove about four miles from Reliance. The Coast Guard cutter made sound contact with the sub, but at 6:20 p.m. at a distance of 4,700 yards, Reliance permanently lost contact with S-28.

“At no time during the approach or the ensuing sound search were distress signals from S-28 seen or heard, nor was any sound heard which indicated an explosion in S-28,” the Navy said.

A Court of Inquiry determined the S-28 sank shortly after 6:20 p.m. Because of the depth of the water, salvage operations were not possible, the Navy said.

The court said the submarine lost depth control “from either a material casualty or an operating error of personnel, or both, and that depth control was never regained. The exact cause of the loss of S-28 cannot be determined.”

Taylor’s “Lost 52 Project,” dedicated to the legacy of lost World War II submariners, said the S-28 was discovered on Sept. 20. But Taylor said the news was only recently made public.

“Based on preliminary video and other documentation,” the website said, “the team currently speculates that the sub suffered a hull failure that resulted in the eventual separation of the bow, causing a near instant loss.”

Another S-class submarine, the S-35, was used as a target and sunk by U.S. torpedoes in 1946, also off the Waianae Coast. That location is perhaps 20 miles away from where the sub was found by Taylor’s group, Hinman said.

Taylor, who has now discovered three sunken U.S. submarines, said he’s hoping to reach out to families of the fallen on the S-28.

“On the last two discoveries we found family members. We met them,” he said. “We’ve been able to share underwater footage and the story, and it’s very cathartic (for them).”