U.S.-Vietnamese teams retrieved two sets of human remains in an unprecedented joint search of Vietnam’s countryside for six American airmen listed as missing in the Vietnam War, officials said. But U.S. investigators said that one of the sets of remains was found buried with the remains of Vietnamese and probably is not American. Bill Bell, a U.S. team leader, said searchers found areas where other remains might be buried and also recovered pieces of aircraft wreckage. After ending their eight-day search through five provinces, the U.S. team left for Bangkok, Thailand, but planned to return to Hanoi on Friday to discuss their findings. The six airmen were downed while on bombing missions from 1965 to 1967 and are among 70 such cases that the United States is investigating.