The Investigator is one of the Town's more powerful roles and necessary to the long-term survival of the Town. The role can investigate a player at night, revealing a crime that they comitted.
As an Investigator a player acts as one of the main advisers to the town with regards to lynching players. The Investigator must provide reports to the Town on whether or not players are suspicious.
When dealing with a player that shares his crime with many other roles, look first if you can exclude the town roles that have the crime too.
Remember that only crimes which have been commited are listed! (e.g. Arsonist only shows "Trespassing" and not "Arson", "Destruction of Property" and "Murder" until he ignites his doused targets)
Another easy way to catch evildoers by asking them to claim their role - if their role does not match the crimes you found, you know they have something to hide! (e.g. You find the crime of "Disturbing the Peace" which could be Crier or Judge. Now if the player claims to be a Sheriff, you know they are Judge, as the crier would have no reason to lie!)
Great diligence is required of an Investigator if the Mafia has a Consort or if a Witch is in play, as revealing their clues too early may result in being role-blocked or fed useless information for the rest of the game. This is especially dangerous if the settings make it so the Investigator don't know they are being controlled by the Witch.
Always leave a last will as an Investigator with important clues that have not yet been revealed to the Town. This information is invaluable. Leave a last will by typing -last will or -lw into the chat and fill information into the prompt-box.
Having an easily readable and understandable format can be very important. For example: