Shortly after the spring thaw in early 1972, teams of FBI agents aided by some 200 Army soldiers from Fort Lewis, along with Air Force personnel, National Guardsmen, and civilian volunteers, conducted another thorough ground search of Clark and Cowlitz counties for eighteen days in March, and then an additional eighteen days in April. Electronic Explorations Company, a marine salvage firm, used a submarine to search the depths of Lake Merwin. Two local women stumbled upon a skeleton in an abandoned structure in Clark County; it was later identified as the remains of Barbara Ann Derry, a teenaged girl who had been abducted and murdered several weeks before. Ultimately, the search and recovery operation—arguably the most extensive, and intensive, in U.S. history—uncovered no significant material evidence related to the hijacking.