After Castle Season 6 aired its mid-season finale, more spoilers and hints are surfacing online with regards to both Castle (Nathan Fillion) and Beckett's (Stana Katic) past!

Before the mid-season finale, Episode 9 "The Disciple" gave its viewers a lot of cliffhangers particularly on the 3XK case.

Remember 3XK? The serial murderer who kidnapped Castle? Well, it seems that his story didn't end on the bridge where he fell after getting shot by Rick.

On Episode 9, a new character was introduced, Dr. Kelly Neiman. As the episode progresses, it turned out that Neiman is somewhat connected to 3XK. The episode ended with Castle and Beckett opening a pen drive Castle got from Dr. Neiman. When Castle played the song "We'll Meet Again," Beckett was shocked, started to shake and almost teary-eyed.

Check out the Top 5 Vital Scenes on Episode 9 here!

After the episode, more speculations surfaced regarding 3XK's relationship with Beckett. Judging by Beckett's strong reaction to the song "We'll Meet Again," there's a great chance there's more to 3XK's story that hasn't been revealed yet.

Castle creator Andrew Marlowe spoke to TVLine about this and said, "Maybe, just maybe, Beckett has a personal history with that particular song that makes her think that it was more than just a general threat. That maybe someone out there knows more about her past than she realizes."

Could this be a major hint on what Marlowe is trying to build up for Castle Season 6 finale? If this major clue proves to be true and 3XK is somewhat really connected to Beckett's past, could it also mean that 3XK is somewhat involved with Senator Bracken (Jack Coleman)?

It could be a long shot, but series creator Marlowe insists that he's planning to bring back Sen. Bracken on the show with one major condition - "only when the perfect story comes up," he told TV Guide.

Remember Bracken's last words to Beckett?

Bracken: "You saved my life."

Beckett: "Yea, well you can't win 'em all, can you?"

Bracken: "I suppose this means I'm in your debt."

Beckett: "Nothing's changed between us, Senator."

Bracken: "Even so. It's a dangerous world out there, detective. You never know when you might need a friend."

If Bracken's story is brought back, the senator's last words to Beckett could mean that he was trying to warn Beckett about a possibility of other "threats" in her life.

Katic also expressed her thoughts to TV Guide saying, "I think that story needs really great closure. They did a really nice job of developing this 'sleeping dragon' story, and [the end] of that should be done with justice to all of the years that they spent developing that."

Could it be that this "perfect story" involves connecting the stories of both 3XK and Sen. Bracken? It's one possibility we'll have to see for ourselves when Castle comes back.

Tune in here for more Castle Season 6 SPOILERS!

Castle Season 6 returns to ABC on January 6, 2014 at 10 pm EDT.