Four three and a half seasons, The Blacklisthas teased us with the question of how exactly Red Reddington (James Spader) is connected to Elizabeth Keen (Megan Boone). Well, he may not be her father... but is it possible that she could still be his daughter?

One popular fan theory suggests that, yes, Liz is in fact Red's daughter — because Red was previously known as Katarina Rostova, a.k.a. Liz's mother.

Perhaps no episode has given more credence to the theory than the fall finale, which aired on Nov. 10. Shortly after a DNA test confirmed that Alexander Kirk (Ulrich Thomsen), who was romantically linked with Liz's mother, was not Liz's father, Reddington — in a moment alone with Kirk — told Kirk "Elizabeth is my daughter." And then, just as Kirk was about to plunge a needle into Red's neck, Red whispered something in Kirk's ear that made him stop short. We don't know what Red said to Kirk, but whatever it was, was apparently enough to make Kirk disappear from Liz's life for good. (Whether his exit was voluntary or not remains to be seen.)

The Blacklist: Is Alexander Kirk really gone for good?

But in a later scene with Liz, Reddington reiterates his previous statement that Liz's father died when she was young, and stresses that he has never lied to her. Probably the only way that both of these statements — Liz's father is dead; Liz is Red's daughter — could be true is if Red is, in fact, Katarina.

Photo: NBC, Virginia Sherwood/NBC

What's known as the "Mommy Theory" would certainly explain why creator and executive producer Jon Bokenkamp refuses to answer definitively whether Red is Liz's father, even when it seems like the question has been resolved within the show. When asked about those seemingly contradictory exchanges, Bokenkamp said: "It's best to let the show speak for itself."

But there have been other hints that point to the theory's validity.

In the Season 3 episode "Cape May," Red has detailed hallucinations of encounters with Katarina, who later disappears on the beach after what appear to be several suicide attempts, telling Red: "You had no choice. It was me or Masha. But you did save me. Through her." Since it's revealed that all of his interactions with the mysterious woman were in his head, some view this as merely an inner monologue, present-day Red being absolved by his former self.

In October, when we asked Bokenkamp whether it's possible that Liz's mother could still be alive, he had this to say: "They never did find her body. That is true. She, as the story goes, walked off into the water and committed suicide at Cape May, and sort of washed away. But it is true that her body was never found."

What do you think, Blacklist fans? Is it possible that Red Reddington is not Liz's father, but her mother?

The Blacklist returns Thursday, Jan. 5 at 10/9c on NBC.