Calyn McLemore, a Memphis native, hadn't been seen since Wednesday.
Family members say they were told he was found with a cell phone. They suspect foul play.
Police say McLemore was working to complete a training exercise with dozens of others around 11 a.m. Wednesday. The exercise involved wading through swamp mud and water that is waist-high to chest-high.
Authorities say the hundreds of acres they searched through were some of the worst they have ever seen, but they were holding on to hope after finding some of McLemore’s gear he had while walking through the course.
Police say they think he could have wandered off-course due to dehydration, and stopped for water.