Silent Witness, series 15, BBC One, preview Dr Harry Cunningham (Tom Ward) takes charge and steers the original investigation, into a triple murder at a farmhouse, back on track. When he goes on a rogue trip to the crime scene, he finds DNA which matches that of a mysterious female serial killer known as The Wraith. But Nikki starts to question his evidence after she meets the parents of a murdered policeman. 'Silent Witness' returns to BBC One on Sunday 1 April at 9.00pm