He's sitting right in front of you ~Bungo Stray Dogs

An anonymous letter shows up to the Detective Agency one morning, the sender requests Dazai and Kunikida to solve a cold case he provided.

The investigation into a missing child and kidnapping case that was closed years ago drives the two detectives apart. One becomes suspicious of the other and trust no longer exists in their relationship.

Does the sender care about the people involved in the kidnapping and missing child case? Is the sender somehow related to it?
Does the sender have some alternate motive as to why he wants to know who this victim is?
Does the sender already know?
Will this case cause one brunette, bandage wrapped detective to become isolated among his friends?