If a prisoner tipped the detectives on the fates of these people, there would be a reward.
The prison system is filled with connections, eyewitnesses, tips, and hints that the cops could have missed. And with the right incentive, be it money, a will to correct past wrongs, or just snitching on a guy they hate, a prisoner can be perfectly willing to aid the police. This is what happened with the case of Susan Schwarz: One con recognized the victim's face on one of the cold case playing cards and alerted the authorities. This caused the police to reopen the case three decades after the crime, examine the evidence, and eventually convict a 57-year-old man from Seattle.
The exact tip the prisoner gave the authorities was not disclosed, but it is believed that the man (then just a boy) was an eyewitness to the crime, and the killer had threatened to kill him too if he ever told anyone. After a hard-knock life and a prison sentence or six, such threats were now old hat, so he had no problem spilling the beans once he learned the case was still open.