There are a total of 9 Case files. You can identify them by the magnifying glass on the map or by going to the 5th tab in the menu titled 'Database' and then go into Case files and set current objective.
By the time you reach the first Comm tower a Case file will open up which is a crime scene where you go in and can gather clues in order to reconstruct what has happened and apprehend the suspect. Once you have completed one Case file the next one will open up for you to complete and so on until you reach the 9th and final one. After this you really will feel like a Crime Scene Investigator.
If a Case file is not appearing try progressing through the campaign checking back after the next main point in the campaign has occurred.
Edit: thanks to ironfelixxx for confirming that the last two case files do not open up until you finish the main storyline.
Below are the solutions to each Case File incase you get stuck.
Case File 1: Coventry Tower Deathtrap
*** Spoiler - click to reveal ***
Step 1: identify the victim
Step 2: scan DNA impact off of the wall
Step 3: scan fingerprints off of the panel
Step 4: review crime scene until the body bounces off the second wall and you will see a red line leading you into a vent with the keycard.
Step 5: open grate and scan the keycard.
Case File 2: Lacey Towers Murders
*** Spoiler - click to reveal ***
Step 1: scan the victims hand
Step 2: scan Black Masks' girlfriend's hand
Step 3: scan Molotov cocktail
Step 4: scan body again
Step 5: scan female victim again
Step 6: scan fingerprint on wall
Step 7: scan shoe polish on floor
Step 8: scan DNA where the table used to be
Step 9: rewind until the person who started the fight has backed out of the door and then scan his fabric
Step 10: scan DNA on counter near the fingerprints on the wall
Step 11: fast forward until the end of the first white bar and scan the mobile phone near the shoe polish evidence.
Case File 3: Jezebel Plaza Fall
*** Spoiler - click to reveal ***
Step 1: identify the victim
Step 2: scan body
Step 3: go to the roof and scan the footprints
Step 4: and scan the briefcase on the pipe
Step 5 apprehend the suspect.
Case File 4: Dixon Docks Shooting
*** Spoiler - click to reveal ***
Step 1: identify victim
Step 2: scan blood splatter
Step 3: scan blood on the buildings ledge
Step 4: fast forward part way through the second line follow the trail to find the camera and hen scan it
Step 5: apprehend suspect
Case File 5: Coventry Fire
*** Spoiler - click to reveal ***
Step 1: scan body
Step 2: scan blood next to body
Step 3: scan fire
Step 4: rewind to start then fast forward to the end and follow the killer up the wall and scan his footprint
Step 5: scan ladder on the floor
Step 6: apprehend suspect.
Case File 6: Helicopter Crash
*** Spoiler - click to reveal ***
Step 1: scan body
Step 2: go to the top of the building and scan the wreckage on the side of the building
Step 3: rewind crime scene to start and follow the red trail and scan the helicopter tail
Step 4: rewind to the helicopter being shot and then follow the red trail to the other building and scan the body. Follow the red trail to start the Deadshot most wanted quest.
Case File 7: Burnley Hit & Run
*** Spoiler - click to reveal ***
Step 1: scan body
Step 2: scan blood spatter
Step 3: scan blood spatter from impact
Step 4: fast forward to the end of the review and follow the red trail to the blood splatter behind the barricades and then scan it
Step 5: apprehend suspect.
Case File 8: Amusement Mile Mauling
*** Spoiler - click to reveal ***
Step 1: identify victim
Step 2: scan blood spatter next to body
Step 3: rewind to beginning and follow the trail to find fingerprints on the air duct.
Step 4: apprehend suspect.
Case File 9: Crime Alley Shootings
*** Spoiler - click to reveal ***
Step 1: scan both victims
Step 2: scan blood spatter
Step 3: fast forward until the red trail goes into the vent
Step 4: open grate then scan the bullet casing
Step 5: apprehend suspect.
If you leave a negative vote or have something that I have missed out please leave a comment so that I can improve this guide and make it more helpful.