Doesn’t work if you just keep responding with no commentThere are some YouTube channels that show (and break down the psychology of) an official police interview. They are instructive in how to think about interrogation and what might go on if a suspect in the Delphi case was questioned.
Police don't simply bring the person in, say "what do you know about this? Nothing? Okay."
Usually the interrogator goes through a very detailed questioning of how or if a person might know the victim, what exactly they did that day, and they go at these points in several different ways so they can see if the story is changing. Sometimes just in the course of this type of questioning, small details are revealed in the suspect's knowledge that he could not be expected to know unless he was at the scene. However, even if this doesn't happen, a second phase of the interview may occur.
In phase two, the suspect may be asked something like "what do you think the killer of these girls might have done? Would he have crossed the creek with them? Why do you think their clothes were in the creek?" For example. And the answers to these questions would be analyzed - the "hypothetical" situation. Often a suspect will reveal details here that incriminate him if there are facts held back from reporting on the case or that otherwise could not easily be guessed.
Finally, there is a specific technique of interrogation used often in the US called the Reid technique. You can see an excellent example of this at work in the Chris Watts murder interrogation, which is widely available to be viewed. In this technique, the interrogator presents a "lesser evil" scenario to the offender to see if he will agree that this other, not-quite-as-bad scenario, was what occurred. In the Chris Watts example the interrogator offered up the scenario that his wife hurt their kids first and he retaliated by killing her. Then they let him talk to his parents and he immediately "confessed" to them this alternate version of events that was not "as bad." But this is not the endpoint. They keep working on inconsistencies in your story and the evidence until they eventually get the suspect to incriminate himself in the actual scenario that occurred. In the Delphi case, if LE knew, for example, that a sexual assault had occurred or been attempted, they might present the scenario that the girls made fun of the suspect and he killed them in a rage. They know this is likely not the motive but they want to see if he will agree to this other, "not as bad" scenario that doesn't involve child rape, and see what real details he might also admit to.
If you want to read about the interrogation of a suspected child killer which actually took several years to bear fruit (the suspect was imprisoned on other charges at the time), read The Last Stone by Mark Bowden.
they can only hold him for so long without clear proof