These cars that might have been involved in murders with lots of forensics are as a genral rule practically torn to bits by the police in search for the last scrap of evidence. Then after being kept at the police yard or the local secure compound, they are scrapped and crushed at a relaible scrapyard, to keep the ghouls from buying them and cashing in on peoples misey. They often have to be kept for years though, due to the appeals process and depending on whether or not they plead guilty or not. The guys at the compound keep them well tucked away and wrapped up usually in a locked garage in case any more evidence is needed. The compund guys know when theyre on to a good thing, they make a fortune from there police contracts, so they make sure no funny business goes on with people poking round the vehicle. These days its genrally 24 hour camera security as well, straight to a server via WiFi.
This relates to cars these days, im sure back in the seventies it was once over with a swab stick for blood, a few Polaroids, then fingerprinted, then that was it.