Ducking & Diving

Season 4, Episode 3:

When an armoured security van is found at the bottom of a lake, a link is made to Michael Dudley who disappeared 17 years previously. Michael's disappearance coincided with the murder of Marie Sinclair. With the discovery of the van, revealing that it was ransacked while underwater, and with Marie's murder remaining unsolved, the team decides to reopen the case. But just when the team finally believes it is getting closer to solving the case, a key witness is murdered.