I’m trying to find a science fiction short story I read in the late 70s/early 80s, possibly in an older anthology: it was likely written in the 50s or 60s.

It’s about time travel and its limits: basically, humanity has found a way to travel backwards in time but not physically, just as onlookers.

So it’s used in forensics to travel back to when a crime has been committed and see who the felon is: I remember one of the characters saying that if you’ve been murdered they cannot prevent that nor bring you back to life but at least they have the means to capture your murderer.

I think they’re using kind of floating balls with built-in cameras for the task but I’m not sure about this, it could be from another story.