A Dark too Far - Lance S. Barron

When Mammoth Cave guide Bob Cetera returns to a research site in Mammoth Cave, he finds the two researchers dead. One lies under a five-ton boulder, and the other, under a ladder across the trail. After the only other witness disappears, the FBI focuses on Bob as their prime suspect. When the investigation stalls, Bob, his fellow guide Keven Neff, and his shift supervisor search the cave for clues to remove the suspicion hanging over Bob.