As a POW himself who was subjected to unimaginable brutality, John McCain would have continued the investigation if he had any credible evidence that American POWs were still being held. (JMHO)
That’s not a statement about whether or not such evidence existed. I simply don’t know.
If you don’t know, please look it all up. There was PLENTY of evidence. IIRC, instead of honestly dealing with that evidence, he pushed some POW family members against a wall.