Interrogation
The police gathered all of the evidence and the boys and were prepared to interrogate them at the station one day. When they all arrived, one by one they took them into the room and asked them a series of questions about where they were that day. They even brought out the evidence that they had.
But the boys were not stupid and knew that there was no hard evidence that was connecting them to the destroyed rock. They just made the claim that they rock like that when they got there. They couldn’t even get them to admit to chiseling their names into the rock surface.