- For the list of items that are rewarded to the player during the Additional Investigation of cases, see Additional Investigation/Rewards.
Additional Investigation is the post-indictment chapter of each case, and is considered to be the last chapter of the case.
For Grimsborough, the player must check up on some of the characters who were suspected but turned out to be innocent. The tasks range from helping out innocent suspects who are in need of something, get more information about the most suspicious, or even catching post-murder findings (such as drug-dealing).
In some circumstances, there will be innocent suspects whom the player will have to help by examining an item before investigating a scene. The player must examine an item before investigating a scene where their help is required. Sometimes the innocent suspect already has something in mind and will tell the player a list of tasks he/she will need help on, so the player will only need to investigate, examine, analyze (if necessary), and report back to the innocent suspect.
Only one suspect needs to be spoken to in Case #1, and two suspects will be spoken to in Cases #2, #51, #52, and #56, but in Case #56, one suspect has to be dealt with at a time. Cases #3-#50 will have three suspects whom the player needs to talk to for the tasks aforementioned. For Cases #53 and #55, there will be three innocent suspects to check up on, but one of them cannot be checked up on until the other two are checked up on first. For Case #54, there will be two legs—one with the killer and one innocent suspect, and the other with another innocent suspect.
For subsequent seasons, the Additional Investigation shifts its focus from helping innocent suspects to investigations that do not involve the murder. In fact, findings made during the Additional Investigation may even hold a greater overall significance to the story's plot than findings made during the main investigation. There is no telling how many suspects need to be checked up on and/or which ones will reward the player for helping them out and/or after the player gained more information about them.
In certain circumstances, there is a requirement for killers to be interrogated once again during the Additional Investigation. As of World Edition, non-murder suspects (quasi-suspects) may also require stars to be interrogated. These quasi-suspects may also be the ones to reward the player.
- In the first 34 cases of Grimsborough, all cases require mastery stars to unlock the Additional Investigation with the exception of the following cases: The Kiss of Death, Innocence Lost, A Deadly Game, The Secret Experiments, The Lake's Bride, The Haunting of Elm Manor, No Smoke Without Fire, The Wollcrafts' Creature, Murder on Campus, and Killing Me Softly.
- Starting from At the End of the Rope, mastery stars are not required to unlock the Additional Investigation chapter, with the exception of In the Name of the Father, Monkey Business and Reap What You Sow.
Starting from the sixth district of Pacific Bay, the Additional Investigation chapters have been named differently in an effort to ensure that all chapters of each case are tied-in with the game's story and its related canon.
- Pacific Bay
- World Edition
- Mysteries of the Past
- New Haven – "A New Beginning"
- Elysium Fields – "All That Glitters..."
- Century Mile – "The Gears of Change"
- Sinner's End – "By Hook or by Crook"
- Coyote Gorge – "Once Upon a Time in the East"
- Crimson Banks – "A Plague on Both Your Houses"
- Wolf Street – "A Run for Your Money"
- Grim Chapel – "Lost Souls"
- Ivory Hill – "No News Is Good News"
- Capitol Peak – "Final Judgment"
- The Conspiracy
- Fairview – "Old Friends, New Beginnings"
- Money Mile – "On Shaky Ground"
- The Greens – "Is This Just Fantasy?"
- Old Town – "The Moment of Truth"
- Maple Heights – "Demons Among Us"
- Misty Grove – "Under the Dome"
- University – "Original Sin"
- Spring Fields – "Seeds of Destruction"
- Airport – "To the Stars"
- Newmark – "Endgame"
- Travel in Time
- Supernatural Investigations
- City of Romance
- Fantasy – "An American Cop in Paris"
Mysteries of the Past
Travel in Time
City of Romance
- The Death of Rosa Wolf is the only case with an optional Additional Investigation.
- Starting from Corpse in a Garden, there will always be one Additional Investigation leg that gets you a burger for completion of that leg. In addition, you will have one leg that rewards you with clothes starting from The Grim Butcher.
- Effective in The Wollcrafts' Creature, the Additional Investigation reward of XP changes to a reward of an amount of coins, given that the majority of players will have already unlocked everything in the Avatar Shop by that point.
- Victor Wollcraft is the only suspect to give three burgers in the Additional Investigation.
- A Brave New World is the only case in which you are rewarded with only a burger and free clothes.
- There Will Be Blood is the only Grimsborough case to have a unique Additional Investigation cover art, i.e., its cover art does not depict images of the suspects to be interrogated during the chapter.
- Welcome to Pacific Bay, The Murder of Amanda Love, The Murder of Mrs Stanbury, The Dead and the Beautiful and The Murder of Napoleon do not include a playable Additional Investigation.
- Starting from Eurasia in World Edition, the cover art for the Additional Investigation is shared by an entire region/district.
- Various Additional Investigation names and/or cover photos are likely references to other works:
- "Ivywood Confidential" is likely a reference to the 1997 American crime film L.A. Confidential.
- "Once Upon a Time in the East" is likely a reference to 1968 Spaghetti Western film Once Upon a Time in the West.
- "A Plague on Both Your Houses" is likely a reference to a quote in Romeo and Juliet, a play by British playwright William Shakespeare.
- "Is This Just Fantasy?" is likely a reference to a popular line from "Bohemian Rhapsody", a song by British band Queen.
- "Under the Dome" is likely a reference to a science fiction novel of the same name written by Stephen King.
- "Endgame" is an obvious reference to the 2019 American superhero film Avengers: Endgame.
- "Kingdoms of Clay" is possibly a reference to a quote in Antony and Cleopatra, another Shakespearean play, which reads "Kingdoms are clay, our dungy earth alike feeds beast as man."
- "Times Are A-Changing" is likely a reference to "The Times They Are a-Changin'", a song by American singer and songwriter Bob Dylan.
- The Renaissance's Additional Investigation cover photo is a parody of The Last Supper, a painting by Leonardo da Vinci.
- "The Art of War" is a reference likely to Chinese philosopher and general Sun Tzu's book of same name.
- "Something Wicked This Way Comes" and "Fire Burn and Cauldron Bubble" are references to originally some quotes in Macbeth, another Shakespearean play.
- "An American Cop in Paris" is likely a reference to the 1951 American film An American in Paris.
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