The third show in the new series, which saw the team examining a woman's mutilated body that had been dumped on the motorway, attracted a 26.2% share of the 9-10pm audience to win the primetime slot.
In a battle of the travel shows, ITV1's 'Greatest Cities of the World with Griff Rhys Jones' came out on top, beating Five's 'Paul Merton in India', with 3.4m viewers and a 14.9% share in the same slot.
In the first part of the new series Rhys Jones spent 24 hours in New York engaging in activities ranging from singing with a gospel choir to cleaning the windows of a skyscraper.
'Paul Merton in India', the follow-up to the comedian's Bafta-nominated China series, drew 1.8m viewers and a 7.9% share of the 9pm audience to Five.
US drama 'Heroes', starring Milo Ventimiglia and Hayden Panettiere, pulled in 2.3m on BBC Two and a 10% share of the 9-9.45pm audience.
It beat Channel 4's observational series 'The Family', documenting the Hughes' family life, which netted 2m and an 8.5% share of the 9-10pm audience.
Later in the evening a new documentary series 'Banged Up Abroad', telling the real-life stories of travellers detained overseas, attracted 1.1m viewers to Five and a 6.5% share of the 10-11pm audience.