US naval historians are a step closer to solving the mystery of why history’s first attack submarine sank with all its crew.

In February 1864 during the American Civil War, the Confederate Navy’s H.L Hunley sank the USS Housatonic by jamming a primitive torpedo on its hull in Charleston Harbour.

It became the first combat submarine to sink another ship, but its eight-man crew had little time to celebrate.

After signalling it had succeeded in its mission, the Hunley disappeared and the crew perished.

Ever since the submarine was raised from the ocean floor in 2000, scientists have worked to determine why the sub never returned to the surface.

Now scientists believe its doomed crew did not release an emergency mechanism that could have helped the vessel surface quickly.

The 454kgs of what are called keel blocks would typically keep the Hunley upright, but also could be released with three levers, allowing the sub to surface quickly in an emergency, said archaeologist Michael Scafuri.

Scientists who removed the century of corrosion, silt and shells from the submarine found the levers all locked in their regular position, Scafuri said.

"It's more evidence there wasn't much of a panic on board," Scafuri said.

The keel blocks don't give a definitive answer, but do provide clues that either the crew didn't think it needed to surface quickly or never realised they were in danger.

The crew moved the submarine through the ocean with a hand crank, and one theory is they were resting on the ocean floor 6kms from shore waiting for the tide to turn to make their journey back to land easier and ran out of oxygen or got stuck.

But there are other theories such as the Housatonic explosion knocking out the Hunley's crew or a ship that sped to help save some of the crew on the Union ship clipping the Confederate sub and crippling it as it tried to dive.

The next step for scientists is to remove more of the corrosion, slit and other material collected on the hull. Over 18 years, they have uncovered nearly a dozen artifacts , reconstructed the faces of the crew members and gained more knowledge about the science behind the submarine, Scafuri said.

"We keep seeing parts that no one has seen in 150 years. All of them add into the mix of what happened and how this sub was operated," Scafuri said.

"After all, we don't have the blueprints."