In The Great Hunt, Fain is freed from his cell in Fal Dara by an unspecified person.
Now, before anyone says "Isn't it obvious?", let's analyse the scene in question.
The door to the outer guardroom opened, spilling in a flood of light, darkly outlining a figure in the doorway.
Fain stood. "You! Not who I expected."
The Great Hunt
It's suggested later on by Loial that it was Nidao and Changu who, corrupted by Fain who had been corrupted by the Shadar Logath dagger, helped him escape. However, we already know that Nidao and Changu had been corrupted much earlier on, noted by Rand when he and Egwene visit Fain. If they were already corrupted prior to that, why would Fain not expect them to help free him?
Secondly, with have Liandrin, whose POV is right before Fain's escape.
Full night already, and there was much to do before dawn. Her orders had been explicit.
What orders? Freeing Fain? Helping the Darkfriends steal the Horn? Spying on the Amrylin? The real reason she's a likely suspect is because of the placement of her POV, which makes me think, immediately after reading about her orders, that she is the one who freed Fain.
Lastly, Ingtar the Darkfriend. We know that it was he who let the Gray Man in, but it's never really mentioned if it was him who freed Fain, or not. He's my most likely suspect; consider this scene:
Rand was shocked, but Loial explained softly. "Shienarians believe we all came from the earth, and must return to earth. [...] And there are never any words except 'The Light shine on you, and the Creator shelter you. The last embrace of the mother welcome you home.' " Loial sighed and shook his huge head. "I do not think anyone will say them this time. No matter what Ingtar says, Rand, there cannot be much doubt that Changu and Nidao slew the guards at the Dog Gate and let the Darkfriends into the keep. It had to be they who were responsible for all of it.
And Ingtar surprises everyone by saying those words on their graves, giving the reason that it was because "they saved Lord Agelmar at Tarwin's Gap".
Or another reason, perhaps because it was him who freed Fain, and (as we know if we're on a re-read) he has already begun to forsake the Shadow again when the Darkfriends stole the Horn, and feels bad for Nidao and Changu taking the blame, so he says the words.
In the reputable WOT:Encyclopedia, footnote 5 of Shadow of Shienar states that it was Ingtar who freed Fain. as confirmed by RJ.
In an interview, Robert Jordan stated that Ingtar was the one who freed Fain. As the cell is dark, it seems that this scene depicts the arrival of Mat and Egwene. (TGH,Ch6)
However it's not sourced, leaving it ambiguous yet again as to who freed Fain.
So, my question is, Who was it that freed Fain in Fal Dara?
It would be great if anyone can find that particular fore-mentioned interview, or any other WoG from the author(s).