On Pretty Little Liars Season 4, Episode 1: "'A' Is for A-l-i-v-e," Mona revealed that she wasn't the one who pushed Ian off the bell tower ... and she doesn't know who did it. Assuming she's telling the truth (which is a big assumption, but let's roll with it), that just makes us more curious than ever about how Ian met his near-demise — and why.

Given that this mystery has persisted for seasons, we feel like the answer might end up being very revealing. But who could the culprit be? At this point, we have three main theories.

Toby. Given that Mona doesn't know who did it, Toby couldn't have done this to cement his place on the "A" team, as some fans suspected. However, it's still possible he pushed Ian to protect Spencer. Though if that's the case, why hasn't he just told her? At this point, there's no need to keep it a secret.

Someone from the "B" team and/or the N.A.T. club. It's still unclear exactly how the Melissa/Shana/Jenna triangle of suspicious activity is linked to the N.A.T. club, but it's clear this group of characters shared many a secret, and Ian was a big part of that tangled web. Perhaps someone involved in this mess was planning on confronting Ian about something, and took this opportunity to silence him for good. Or, heck, given that Melissa seems to have something of a leadership role in this contingent, maybe someone was doing it specifically to protect Spencer at Melissa's request. (Melissa was in the hospital at the time and therefore probably didn't do the physical shoving.)

Red Coat. Sure, the culprit wasn't wearing red, but that doesn't mean it couldn't have been the mysterious Red Coat, whose agenda is still unknown.

Who do you think pushed Ian? Sound off in the comments below.

Find more Pretty Little Liars spoilers here.

Catch the next episode of Pretty Little Liars on Tuesday, June 18, 2013 at 8 p.m. ET/PT on ABC Family.