Sean Flynn and Dana Stone were last seen on April 6, 1969 on Route One somewhere near the Viet Cong front in Cambodian. Dana and Sean were American photographers covering the Vietnam War via motorcycle when they were captured by the Viet Cong at a Cambodian check point. The photo above was taken roughly two hours before their capture.
Flynn, a former actor, singer and Duke student, and Stone, a former logger, model, sailor and gold prospector, had no prior formal photographic training before arriving in Southeast Asia. Their appetite for risk and adventure put them in great situations to take photos established them as some of the most famous photographers of the war. It also brought about their demise.
Zalin Grant wrote a great piece discussing their lives and their disappearance available here.
Men's Vogue has a slideshow documenting Sean Flynn's.
Francis Ford Coppola and John Milius based Dennis Hopper's character in Apocalypse Now, pictured below, around Flynn and Stone.