MAM..I am thinking of A Friend in Deed...but I don't know...could you be a litte more specific..and maybe we could help you out................and by the way....HI!...I don't think I have seen you on this forum before...but welcome..
In "Murder Of A Rock Star" (one of the "new" episodes), the killer is a big-shot defense lawyer, who "has never lost a murder case", so he knows a bit about forensics and murder investigations -- he's also a personal friend of the mayor. And for these reasons, Columbo is ordered to share his case file with the killer.
In "A Friend In Deed" (an "old" episode), which Cassa remembers, the killer himself is Columbo's boss, so he just plain orders Columbo to show him the file.
I read a more detailed write up of the plot of "Murder of a Rock Star," and that sounds like the one to me. Many, many thanks to all who helped solve this one.
In a Trace of Murder the killer is somebody on the forensic team. I think he offers to help Columbo. Can't really remember this one. Only seen it once.