More than 83,000 Americans are still missing from past conflicts, according to the Joint POW/MIA Accounting Command. At any given time, there are more than 1,000 files under investigation. The recovered remains of more than 1,800 Americans have been identified since the accounting effort began in the 1970s.

Richard Rivenburgh went missing after Khmer Rouge gunboats captured the S.S. Mayaguez in May 1975 in the Gulf of Thailand, about 60 miles off the coast of Cambodia. The vessel was taken to Koh Tang Island, and on May 15, 1975, the Air Force sent in 11 helicopters with troops to secure the island and rescue the Mayaguez crew.

As the helicopter carrying Rivenburgh neared the island, it was shot down with 26 men on board. Half were rescued at sea, leaving 13 missing. In 1995, all but four of the missing were recovered.

Lt. Col. Thad Trapp, who commands the 2nd Battalion, 9th Marine Regiment — the same Marine Corps unit that Rivenburgh was assigned to — said it's important for troops fighting in current wars to see the U.S.