I’m very much into European and especially French political thrillers of the 1970s - Costa Gavras etc. I think I’ve seen most them and it’s possible that my mind has constructed one out of them all 
Anyway, I seem to remember seeing a movie years ago which is either late 1970s or early 1980s where right at the beginning someone - probably a politician - is shot in a very public place with lots of people around. We then see the police investigators watching hours of video footage trying to identify as many individuals and looking for anyone who might be a suspect. This is basically my strongest memory of the film: police officers sitting in a dark room playing old school videotapes back and forth (early CCTV, press tapes, private videos - anything they can get their hands on). We see the assassination and its surroundings again and again, from different perspectives.
At the end, there’s a shoot-out in a high-rise building. Someone is shot in an elevator.
That’s pretty much it, I’m afraid. Who knows, maybe I’m imagining it all. 
It’s neither I as in Icarus by Henri Verneuil nor The Assassination by Yves Boisset.