Although people are thinking that Spencer is A that gets me thinking about the night that Aria dropped the story that Ezra wrote about Ali. What if Aria is A? Like she's the best liar and she said to Ezra "I'm going to read this cover to cover to see what was so important to ruin our relationship" (something like this). But, Ezra causes her to drop it. Maybe the person picking up the story was Aria. She even said how she wanted to read it, so it must be important to her. CONT.
So lets talk about Jason. We all know he has a alcohol/drug problem. On the night of Ali's disappearance, lets say he faked his drugged up condition and helped Ali escape that night? Lets say he has known all this time Ali has been alive and helping her find the person after her, hence why he offered the money. Lets say he pushed Ian off the bell tower, explaining the body bag in his front garden.