A lawsuit forced the Navy to release files from the accident investigation. A trove of recently declassified files on the tragic 1963 sinking of the nuclear-powered attack submarine USS Thresher confirm the U.S. Navy didn’t cover up the mysterious accident—and, in fact, there was no single event or error that caused the sub to sink. Last year, a retired Navy submarine commander won a lawsuit forcing the service to release its report on what happened to the Thresher, which sank during diving tests in April 1963, claiming the lives of the entire 129-person crew. The Navy has since released several sets of documents that shed new light on the sinking.