Pretty Little Liars: What Does "A.D." Have to Do With Mary Drake and Dr. Rollins?

As we know, the whole premiere focuses on finding Charlotte's killer in order to keep Hanna safe from harm. After 24 hours of detective work, the team tosses Ali into the ring as their suspected killer. Shortly thereafter, Dr. Rollins visits Ali; he tells her he knows she's the one who murdered Charlotte. The only way Dr. Rollins could have found this out is by being "A.D." or at least being close to them.