Did anyone investigating the crime simply ask each person questioned to tie their shoes, or simply tie a sturdy knot? I think that would have been the first thing I had them do if I felt it were at all relevant. Think about it... If someone was guilty and put on the spot, they wouldn't know if it was better to tie a knot the way they normally do, or try to do it differently. I might approach it by saying, "I am going to ask you to perform a simple task, and you have 10 seconds to complete it. Do not ask questions and do not over-think it; just do what immediately comes to mind." I would probably have them repeat the task a few times. I might even phrase the question in a manner to make it appear that time/speed in performing the task was more important than the type of knot, just to ensure that they are not over-thinking it. While it wouldn't definitively prove or disprove anything, it could be quite telling.