Mary Drake

• Why she was suspicious: At the beginning of season seven, it really seems like Mary Drake is running the show. She's the mother of Charlotte, and she's very angry about her murder, so there's motive. We also see her plotting with Elliott Rollins/Archer Dunhill to take all of Alison's money.
• Why she's innocent: Mary saves Spencer in the finale, right after Jenna shoots Spencer. Mary then tells Spencer that she is her real mother as she is bleeding out. Not only is it far-fetched for Mary to have tortured her own daughter, but the real "A.D." drags Jenna away while Mary stays with Spencer.

Jenna Marshall

• Why she was suspicious: Jenna mysteriously returns to Rosewood [3] and meets up with Sara Harvey. She lies about the reason she's back, and we later find out she wants to know what exactly happened to Archer and Charlotte. After Sara Harvey's death, it's easy to implicate Jenna, who'd been working with her.
• Why she's innocent: At the end of the episode, Mary Drake knocks Jenna out. We see a shadowy figure drag her away. When she wakes up, she's in a car with a person in a black hood in the driver's seat. Jenna then says out loud that the driver is "A.D.," meaning it couldn't possibly be her.

Noel Kahn

• Why he was suspicious: This rich boy has always been pretty sketchy. It doesn't help that he comes back to Rosewood to meet up with Sara and Jenna. In the wake of Sara's murder, Hanna is determined not only to prove that Noel is behind it all, but that he's also the second child of Mary Drake. And let's not forget: there's video proof that he was in the dollhouse where the girls were tortured.
• Why he's innocent: In the seventh Summer finale, Noel dies by beheading himself [4] with an axe! At the end of the episode, "A.D." is about to go for a drive and is very much alive. So there's the end of that theory.

So who is "A.D."? It could be someone we never met before or another blast from the past. Either way, we've taken three huge suspects off the list.