From SA Mafia Wiki
A Forensic Investigator is a Town investigative role. They have a Night Action to analyze dead players to discover who else targeted them. The extent and power of this ability depends on the moderator that made the role, but typically the Investigator is given a list of all the players who targeted their dead player, in no particular order, from the start of the game.
The obvious problem of the Investigator using their power on the Night 1 Mafia kill is not easily resolved, and is one of the reasons why it is not a commonly-used role on SA.