all police/crime scene detectives have chunks of dna and blood tests. Being a detetctive myself, I can say that police get tested more than any other human or animal to roam this earth. The reason for this is because contrary to beliefs, the first person to be questioned is the police/investigation force. Why? This is to ensure that these people are not related or friends with anyone associated who may be a suspect in the case. Also, it is because, again contrary to beliefs, that someone in the justice system is usually not suspected of being a suspect.
so adachi, yu, and ryotaro would've been the first three people questioned.
Even though it seems like the last two are, you know, the ones doing the questioning? Maybe it's because they seem to be half of the police force in Inaba, but I get the feeling that Dojima is the one leading the case.
there are a lot more. Remember, when Adachi was arrested, Dojima was not allowed to question him so other authorities questioned him. These people would've done the same for this letter. Another thing to note, dojima and adachi are not the ones who identify evidence, they are just crime scene investigators. --- The official Drake, Toro, and Sir Daniel of the PSABR boards.