Songbird.
McCain gave testimony and broadcast to his VC captors. He gave testimony in captivity admitting to war crimes in exchange for special treatment.
We received only half the number of POWs from Vietnam as expected from Intel reports. McCain spent his office career shouting down and destroying anyone who tried to launch probes into reports of POW/MIA Americans in Vietnam. Reports persist up into the 90s and even some of the 2000s of American POWs giving care to Vietnamese political prisoners who were later released.
Depending on who you talk to… The ones who didn't return but had a high probability of being in POW camps were those from where McCain was moved to receive special treatment.