Credit: ABC Family

Big "A". Red Coat. The sicko in charge of it all. Whatever you want to call shim, the Liars' greatest threat is still a mystery on Pretty Little Liars, which means every episode is a new piece of the puzzle. Who do we think "A" could be after Season 4, Episode 9: "Into the Deep"? We've gathered our top suspects.

3. Alison Dilaurentis

Why she might be Big "A": Ali is always on our list, though this week we've moved her down a bit, for once. Still, there are plenty of clues pointing in her direction. She could easily be the one who wanted Jenna to keep quiet about what she knows, and there's no reason she couldn't be the one who'd kept an apartment in NYC. And all of this stuff about CeCe is making us wonder about Ali, too. We know they were close and that Ali betrayed her, so there are two scenarios that would make sense to us: A) They made up and now CeCe is helping a hidden Ali, or B) They never made up, and Ali is using CeCe as a convenient scapegoat.

Why she might not be Big “A”: Well, that whole "maybe being dead" thing is always a factor...

2. Wren Kingston

Why he might be Big "A": We've talked about Wren as an "A" suspect in the past, and Mona checking into Radley this episode just makes us even more curious. There has to be a reason that she wants back into her old room — our guess is she's either following Big "A"'s instructions, or trying to track Big "A" down. In either case, Wren is one of the people most associated with Radley, so we have to wonder.

Why he might not be Big “A”: As we've always said, at this point we still have no idea what Wren's motivation for being "A" is. Unless, of course, he’s the mysterious beach hottie...

1. Melissa Hastings

Why she might be Big "A": Melissa is back on our suspect list in a big way because of how important Jenna and Shana became again. We've heard they are scared of Melissa, and we'd easily believe that Melissa would attack Jenna to keep her quiet  — and given their preexisting relationship, it's not hard to imagine how Melissa could lure Jenna out to the lake to begin with. Plus, her internship in England would be a good lie to cover up nefarious doings in Rosewood, it's easy to imagine Melissa is good at playing the piano, as we saw "A" doing at the end of the episode, and we know that the person Nigel was calling has a PO Box in Philly, a town Melissa has spent a lot of time in...

More broadly, Melissa being "A" would explain "A"'s hot and cold relationship with the Liars, where despite torturing them, "A" sometimes seems to be helping them, too. And since we know Melissa and CeCe have a history, we'd also believe that Melissa is forcing Shana and Jenna to frame CeCe.

Why she might not be Big “A”: Melissa has been a top "A" suspect one too many times; doesn't she seem a bit obvious at this point?

Of course, these aren't the only people we have our eyes on. For instance, CeCe is obviously highly suspect, though at this point she seems too obvious. And we're starting to wonder if Mona could possibly be playing everyone, pretending to answer to someone when really she's in charge (it doesn't seem to quite fit, but it's an interesting idea). In Rosewood, anything is possible.

Who do you think "A" is? Sound off in the comments below!

Catch the next episode of Pretty Little Liars on Tuesday, August 13, 2013 at 8 p.m. ET/PT on ABC Family.