Shattered

This morning when we woke up I heard a crackle sound every so often and couldn't figure out what it was. It kind of sounded like tree branches hitting a window, but we don't have any trees that close to the house so it obviously wasn't that. I got up and started to investigate. First looking out our bedroom window, nothing unusual that I could see. Then I went downstairs into the kitchen to look out our sliding glass door, nothing there. So then I ventured downstairs. Right as I turned the corner into the room, I saw one of our windows down stairs was just shattered and hanging there with a big hole in the middle. I ran straight upstairs to tell Todd what I had found. The window is double paned, but only the outside pane was broken. It looked like a rock or some object was thrown at the window, but there was no rock that stood out near the window. So there you have it, a mystery as to what broke our widow. Any ideas?

A view of the window from outside.

A view of the window from inside.