"So I'd be interested why?
"The supervising detective just got a call a half hour ago?"
This is The Broken Window by Jeffery Deaver.
Context: Rhyme received a call from a detective, in which the detective told him that there had been a murder
in one of the neighborhoods of the city. The murderer had been caught. Rhyme asked him why he would be interested
in a crime whose doer had been arrested. The detective told him that the perpetrator was his cousin, Arthur Rhyme.
In the underlined sentence, why is a question mark used here?