Sherlock and Joan look into past murder cases to solve a recent one in tonight’s episode of “Elementary.”

According to the synopsis for Season 6, episode 7 of the CBS series, Sherlock (Jonny Lee Miller) and Joan (Lucy Liu) hunt a serial killer after a murder rocks their inner circle. That murder, as revealed at the end of the last episode, was the death of Hannah’s (Liza J. Bennett) roommate named Maddie (Comfort Clinton).

In the promo for tonight’s episode, Sherlock says that Maddie’s death isn’t a homicide, but a part of series of murders carried out by a serial killer. “This is not just a homicide, this is a serial killer announcing himself,” says the British sleuth.

In a sneak peek from the installment, Joan and Marcus (Jon Michael Hill) find out that a certain Ashley Jenkins was killed in the same manner as Maddie. “Ashley Jenkins, she was killed in East Rutherford five years ago. She never went missing. She was found strangled in her home like Maddie,” Joan tells Sherlock and Captain Gregson (Aidan Quinn).

Asked if Ashley’s case was closed, Marcus says that the police in New Jersey arrested Ashley’s husband, Graham, for her murder, and was later convicted by a jury.

“[But] at his trial, Graham Jenkins said he came home and found his wife’s body,” Joan shares. “There was plastic tubing around her neck, again like Maddie. He said he saw a man running out the back door just as he was walking in.”

Marcus says that Graham got a look at the man who he thinks murdered his wife but no one was interested in his story.

When Sherlock says that he’s interested in hearing Graham’s story, Gregson says that the two of them are going to New Jersey to talk to Graham.

In another sneak peek from the episode, Sherlock tells Joan that Graham got a letter that reads “You are never meant to take the fall” while he’s in jail, and believes it’s from the man who killed his wife.

“Of course it wasn’t signed, and it was so woefully mishandled that there was no chance of forensic analysis revealing the author’s identity,” says Sherlock. “If it is genuine, if the killer did send it, it was likely to torment Mr. Jenkins. [But] what intrigued [me the most] was the postmark.”

When Joan looks at the postmark, she learns that the note was sent from Vietnam. Sherlock, however, admits that he’s having a hard time finding an Asian connection to the killer.

“Elementary” Season 6, episode 7, titled “Sober Companions,” airs tonight at 10 p.m. EDT on CBS.