In 1995, researchers discovered the wreck of the Confederate Navy’s submarine, the H.L. Hunley, the first combat submarine in history to sink another ship. In 2000, they were able to raise the sub, including the remains of the eight sailors aboard. But the Hunley presented a mystery—soon after jamming a rudimentary torpedo into the side of the U.S.S. Housatonic on Feb. 17, 1864, the submarine also sank, though there are no signs that it was attacked or damaged. Now, reports Brandon Specktor at LiveScience, a new finding from the sub may give some clues.

Since being raised, archaeologists and conservators have painstakingly excavated 1,200 pounds of concretion—rock-hard silt and sand that accumulated on the 40-foot-long, sausage-shaped craft as it sat four miles off the coast of Charleston, South Carolina, for 150 years. Researchers recently worked on removing the concretion from and conserving eight cast-iron keel blocks, weighing about half a ton total, that had been used to steady the submarine. But they also found the largest blocks were connected to a quick-release mechanism, meaning if there was any trouble the crew could eject the blocks and quickly rise to the surface. Recent work on the sub shows that the three levers of the release mechanism, however, were never engaged and the keel blocks are all in place, meaning the crew never tried to use the safety feature. That suggests that they either did not think they were in trouble or were incapacitated before the boat went down.

“As a diver, your first instinct if you’re in trouble is to get to the surface by releasing your weight belt, and it’s part of your training,” Johanna Rivera, a conservator on the project, tells Bo Peterson at The Post and Courier. “The keel blocks serve the same purpose, so it appears there was no sense of panic (among the crew)…[The finding] is an extra layer of complexity as to what really happened.”