It is part of a criminal proceeding, they don't just want to question him. They want to question and arrest him, they must question him again before they can a

It is part of a criminal proceeding, they don't just want to question him. They want to question and arrest him, they must question him again before they can arrest and charge him. They already discussed the case with him once, this second time is more of a "convince us not to arrest you".