He Needed to Find Evidence
He had a suspicion about what the boys had done, but now idea how they had done it. He hoped that he could find evidence of their deed in the house somewhere, but Marty knew that that would probably not be easy.
If they were smart enough, they would have already gotten rid of everything as soon as the deed was done. But Marty did not estimate them to be that smart and so he figured to still have a chance. And he was very much right about this…