ID: Ask him if he knows anything about the murder, despite what he's said.

"DM, we both know you work with your boss like, every day, right?" "Yeah, that's true." "So you have to know something about this murder." You'd reveal what you and PD know already, but who knows what DM would say to his boss. "Look, Dame, I really don't know anything! Ye've got to believe me, Mr. M has been busy with something all night. I'm not even sure if he's in the building." You hate to admit it, but you think he might be telling the truth.