I think the feds were hoping that Nick could point the house out because it would stregthen there case.
The murder of the Spilotro brothers was a really brutal crime scene. I have copies of the photos of the brothers in the half dug grave with the cops digging the bodies out and a few photos of them in the mourge. There faces were so black and blue they looked like extra's from a zombie film.
I think it happened so fast that Michael didn't have time to react. They both knew that it was a 50/50 chance of them coming out alive.