I can't help but surmise that Klaus would have corroborated that Wharton was in his employ, then disappeared after the disappearance of his daughters. That much he would be compelled to admit, no?
When you think of garbage, think of Hakeem!
Folks, calm down! This is not the last chopper outta Saigon!

John states that he wants to die and that he wants it to be over with. He's finely tuned to all the pain and suffering in the world and feels it as though it's his own and he can no longer bear it.
They have absolutely no proof. If they stated that John showed Paul in a vision what had happened to the girls, they'd likely be in the room next to Percy in the mental institution before they finished their sentence.
Although they can prove that Klaus Detterick knew Bill Wharton, they cannot prove that Wharton murdered the girls or that he was at the scene of the crime and Wharton died before Paul discovered the truth so they could not get a confession from him.
John was a black man in the Deep South in the 30's. He was guilty until proven innocent, not vice versa. He was not condemned to die by a jury of his peers, he was condemned to die by people who saw him as an animal, a lesser being due to his race.
In the book, Paul speaks to a journalist who believes that John did kill the girls, was remorseful but couldn't help it due to "black impulse" and that his "black nature" meant he was no better than an animal. That was the attitude there.
Ironically, in the book, the police chief, who is a massive, absolute racist and decided John was guilty based on his skin colour died of a heart attack while having sex with a teenage black girl over his desk - one he had bribed for sex in order to release her for a minor offence.

John was a prophet.
Like Jesus, he forgave the executioners. It was fate.

That makes sense.
I will make you famous.

They had no legitimate proof and this was the deep south in the 30s where a black man was suspected of killing two white girls, surprised he wasn't hanged.