Covering their tracks
The group went back to their homes and looked around for anything that could link them to the rock. Some of them even had pieces of it in their bags and at home. It was a sort of souvenir they collected from that day. They discarded those pieces of shattered rock immediately.
After that, they felt a little bit better, now it would be almost impossible for the police to nail them to this crime. But the police were not done with their investigation just yet. They knew what group they were focusing on now and they were determined to see that they get punished.