Box cutter
When Sam is trying to piece together the newspaper article in Brooke's office, it shows somebody cut a knife or box cutter that's hanging from a rope or something. I'd assume it was Brooke since she was on her way to her office, but I can't figure out a reason why she would have it in the first place. She was not the killer. Im assuming since Gayle was the killer, it would have to have been Her, but she was down stairs at the auction the whole
time. Any ideas???