You go over to the competent investigator and introduce yourself. "Hey there," you say. "I'm Kallen Stanner, warrior of Stormwind. Is there anything I can do to help?" The investigator looks up at you. "Thank you for the offer," he says, "but we have things under control here. I'd advise that you return to your usual business here." He resumes his work.
Return to your business in Elwynn. Looks like they have everything worked out here. Look around for clues on your own. The investigators seem to have things under control right here, but there might be something nearby they're missing. Stick around. There must be something useful you can do here.