promising 23-year-old law student was stabbed to death in the tiny apartment where his friends raul gupta a grad student and gupta's girlfriend, taylor gold a biomedical engineer lived. they started the evening bar hopping but ended it in police custody. >> here, can you hop up for a minute? >> detective paula hammel thought it would be an open and shut case because gupta told officer skeba that he did it and why. >> i caught my buddy and my girl cheating. i killed my buddy. >> as gupta sobered up he changed his story, now insisting he had no idea how mark wall was stabbed. >> i did not attack mark. i just want to put that on the record. >> and strangely though taylor gold insisted she didn't remembered she couldn't say for sure it wasn't her. >> did you kill mark? >> i don't think so. >> after hours of questioning detective hammel brought them together. >> i did not stab him. >> i don't think i did either. >> this conversation could decide who goes home and who goes to prison for murder. >> richard schlesinger from 48 hours joins us live. richard what do the forensics in this case tell u