Castle (Nathan Fillion) and Beckett (Stana Katic) might become involved in a national investigation in an upcoming episode of Castle season 6.

Beckett To Face Off Against Mother's Murderer Again?

Castle fans will be meeting the estranged sister of Captain Gates (Penny Johnson Jerald) in episode 19, according to multiple reports. Gates' sister is Elizabeth Weston (guest star Salli Richardson-Whitfield), a U.S. Attorney who becomes a key player in Castle and Beckett's latest case.

"We end up with a victim who it turns out is a confidential informant for the U.S. Attorney's office," Castle creator Andrew W. Marlowe told TV Guide this week. Gates' relationship with her sister is not exactly a close one. "Gates doesn't talk about her," Marlowe teased.

Castle & Beckett Battling '70s Mobsters & Ninjas?

In other Castle news, Katic recently spoke to TVLine about a fan's question regarding why Beckett never wears her engagement ring.

"The important thing to remember is that Beckett is a detective, so she can't wear things that will identify her, she can't wear things like that on her hands that would distract from the conversation, that could be stolen, that could possibly hinder her when she's shooting or hand-fighting," Katic explained.

She added that Beckett is very protective of the ring since she loves it so much, so she would only wear it on special occasions.

"But having it on display at the work place is not appropriate, especially as a detective," she noted.

Castle season 6 airs Mondays at 10 p.m. on ABC.

Watch the promo for the next episode of Castle here:

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