Danny Williams had long been haunted by the consequences of Matt's actions. And it seems that even though his younger brother had been dead for years, his crimes are still affecting the Williams family as they are once again targets of an FBI investigation in episode 15 of "Hawaii Five-0" season 6.

"Hawaii Five-0" fans last saw Danno's (Scott Caan) brother alive way back in season 1 when Matt visited the island in episode 18. At the time, he was running from the FBI for laundering money for a drug cartel. He was later revealed to have been murdered in season 5.

However, it seems the FBI is keeping Matt Williams' case open as the agency continues its search for the money he stole.

According to a synopsis for episode 15, "Ke Koa Lokomaika'I" or "The Good Soldier," the FBI questions Danno's mother, Clara (Melanie Griffith), about the missing money. Meanwhile, McGarrett (Alex O'Loughlin) works with an autistic young man who might have information that will help solve his friend's murder.

The upcoming episode's trailer show's more of "Five-0's" hunt for the murderer. The sneak peek begins with Danno opening the door to an FBI agent looking for his mother. Viewers are given glimpses of Danno's mom while Steve wonders what the FBI would want with Clara. The clip also shows the Lieutenant Commander checking a hole made in a wall. A suspect about to shoot Grover (Chi McBride) is taken down by Kono (Grace Park), prompting him to shout his appreciation for the sharpshooter. There are also scenes of a man being thrown out a window, the team checking what appears to be a bank vault and Steve helping a bloody Chin Ho (Daniel Dae Kim) out of a building.

Why the FBI is targeting Danno's mom and why Matt's case is still active does raise questions. Did Matt perhaps left some of the money he stole with her?

Aside from Clara Williams, another fan favourite character might be coming back into the lives of the "Five-0" team. Reports have it that McGarrett will be confronted by a new character over several calls Catherine Rollins (Michelle Borth) allegedly made to him. The last time fans saw the character, she left Steve for a secret mission just as he was about to propose to her. Now it seems viewers will finally know what she has been up to all this time.

A new episode of "Hawaii Five-0" season 6 will be aired on Feb. 19 on CBS.