SR was murdered on July 10th. Wikileaks released them on July 22. But Guccifer 2.0 was shopping them around in late June. I can't find an exact date.
News of this would have gotten back to the DNC almost immediately. If it were me, I would have examined the leaked files for the latest timestamp to determine the approximate time when they were stolen and then looked back through the building access logs (assuming there are any; a modern operation like that would probably use swipe badges) for anyone who stayed late one evening who normally doesn't.