Warning: There are spoilers ahead for "Game of Thrones" season six.

The latest "Game of Thrones" episode ended with a fiery surprise from Daenerys Targaryen, but before that a pivotal scene took place in Castle Black. Jon Snow was sent a threatening letter from a Bolton rider and sealed with the Bolton's flayed-man sigil. Though the letter was signed by Ramsay and all signs pointed to the new Bolton lord being the true author, many fans believed that the letter was written by another hand.

Well, we know for certain that it was definitely written by Ramsay.

"Game of Thrones" director Dan Sackheim confirmed with Tech Insider that there is zero ambiguity when it comes to the letter. "This I can tell you without any concern [of spoilers]. That letter was written by Ramsay," Sackheim told TI over the phone.

He explained why Ramsay sending the letter was so significant.

"It's the ignition, or trigger, in terms of them going through the process of taking Winterfell back," Sackheim said. "How they're going to take Winterfell back, I can't say. But it's the beginning of that process."

So why did fans believe that the letter was sent by someone else? For starters, the book series' version of this letter arriving to Castle Black has a lot of different circumstances and clues. Since the show and books are now following separate canons, the fan theories related to the book letter could hold up.

For show viewers, there was some hope that the letter came from Sansa. This would make sense from a motivation standpoint. At the beginning of the episode, Sansa was trying to convince Jon to help her take back Winterfell from the Boltons. He was adamant about staying away from the fight — until this letter arrived. People believed that Sansa was "playing the game" by writing a letter she knew would antagonize Jon into attacking.

Others suspected Littlefinger was somehow involved, since he's proved to be a sneaky player in "Game of Thrones."

"It could always still be Littlefinger," Redditor tatertatertatertot wrote. "He's the intellectual Ramsay on the show -- he can do anything with nothing. Fake Bolton riders and seals? Sure. It's Littlefinger. Why not?!"

Others seem inclined to believe Littlefinger's involvement, too.

"I suspect Littlefinger has been coordinating the Northern houses," Redditor Rhyal wrote in another thread. "I think he setup [sic] Sansa to trigger a northern revolt. When that didn't work, he coordinated with the Umbers to deliver Rickon into harms way (motivating many houses into action)."

We know Littlefinger often has secret plans that aren't revealed to the viewers until after they've been executed — i.e., Joffrey's murder — but based on Sackheim's comments, this isn't one of them.

Fans will have to stay tuned to see exactly how Jon and Sansa work their way toward taking back Winterfell. Episode five, "The Door," airs on Sunday, May 22, at 9 p.m. EST on HBO.