'The Walking Dead' finally reveals one of the biggest mysteries fans have been wondering since the show began

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Warning: There are huge spoilers ahead for "The Walking Dead."

Sunday's episode of "The Walking Dead" saw the return of Negan reuniting with Rick and his group for the first time since the season seven premiere. While their reunion was anything but rosy, fans were in for a shock when the show ended up delivering an unexpected reveal that had nothing to do with Negan, the Saviors, or any other future characters.

Near the end of the extended 90-minute episode, Rick revealed the answer to one of the biggest mysteries the show has had since its first season: Who is the true father of his daughter Judith?

Last chance to head back before spoilers.

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Old school "The Walking Dead" photo from season two with Rick's old pal Shane and his wife Lori.

While telling Michonne that they need to accept the way the world is right now, with Negan in it, Rick told Michonne that he previously had to accept that the daughter who he has been raising isn't his.

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Yes, Rick has known all along that his daughter with Lori actually belonged to his best friend Shane.

Here's the speech Rick gave to Michonne during the big reveal:

I had a friend. I don't talk about him. He was my partner.

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He got Lori and Carl to safety right after it all started. I couldn't. I was in the hospital. I didn't know what was happening.

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My friend, his name was Shane, him and Lori they were together. They thought I was dead.

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I know Judith isn't mine. I know it. I love her. She's my daughter, but she isn't mine. I had to accept that. I did. So I could keep her alive. I'll die before she does and I hope that's a long time from now, so I can raise her and protect her and teach her how to survive. This is how we live now. I had to accept that too so I could keep everyone else alive.

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Both of Rick's children, Carl and Judith.

Rick's explanation to Michonne echoes what Lori said to Shane after he found out she was pregnant: that their child would always be Rick's.

"Even if it's yours, it's not gonna be yours," Lori told Shane in season two, episode seven. "It's never gonna be yours and there's nothing you can do to change that."

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No one was expecting that reveal to come on Sunday's episode, but it is one that fans like to discuss.

In 2012, TV Guide asked comic creator Robert Kirkman if we may ever learn the identity of Judith's real father and he hinted that Shane was indeed the dad.

"I'd prefer to keep that kind of thing somewhat ambiguous, but I will say if you've watched this show, it's definitely much more likely that it is Shane's baby, but you never really know," Kirkman said at the time.

Though his character has been dead on "The Walking Dead" for years, even actor Jon Bernthal, who played Shane, guessed that Judith was his daughter several times.

The actor told fans Judith was "100% Shane's" at Wizard World Chicago Comic Con in 2013. During an appearance at October's New York Comic Con, the actor told fans the same when he was asked Judith's paternity.

Whether or not Bernthal actually knew the truth is another thing entirely, but it's a fun question he loved to answer regardless.

As fans have pointed out, it logically made more sense that Judith was Shane's child. It's estimated that Rick was in his coma for four to five weeks and then a few more weeks until he was finally reunited with Lori, his son Carl, and Shane. By that point, fans believe Shane was with Lori for the better part of two months.

When the question was asked on Reddit over a year ago, user TsarNab came up with a pretty convincing timeline in which it was determined that Rick returns to his family on day 60 of the outbreak. It's on that night he has sex with his wife. Eight days later, Lori takes a pregnancy test that's revealed positive. It's determined that eight days later would probably be too soon for an over-the-counter pregnancy test to reveal positive results and so it was largely assumed by fans that Lori probably conceived a while ago and that Rick wasn't the father. You can read TsarNab's full breakdown here.