Almost 10 years after the brutal murder of a British family at a beauty spot in the French Alps, the case remains unsolved.
Former investigators and the brother of one of the victims - once the prime suspect in the case - tell the story of the mysterious shooting and the sprawling police investigation that continues to this day.
Twisting and turning through a list of increasingly bizarre theories and leads - family feud, crime of passion, international espionage, political assassination - this true-crime documentary series examines the intrigue around the case and asks why it remains unsolved a decade later.
On September 5th 2012, police arrived at a picnic spot in the woods near Lake Annecy to find British computer engineer Saad Al-Hilli, his dentist wife Ikbal, and her mother Suhaila all shot dead.
Nearby, on the road, lay the body of local French cyclist Sylvain Mollier. Saad and Ikbal's eldest daughter Zainab, seven, had also been shot, pistol-whipped and left for dead. Cowering underneath her mum's skirt in the back seat of the car was Zainab's little sister Zeena.
Investigators quickly turned their focus towards the UK - pointing the finger at Saad's brother Zaid, as a suspect in the case. Caught up in a media whirlwind, Zaid was eventually arrested under suspicion of conspiracy to murder and accused of organising the killings of his brother and family over an inheritance dispute.
Zaid was later cleared and released but continues to battle public suspicion to this day. Telling the full story for the first time, Zaid is joined by former French prosecutor Eric Maillaud, retired British senior investigating officer Mark Preston, who formerly led the investigation for Surrey Police, and journalist Tom Parry, who covered the case from day one.
Close friends of the victims complete the cast to tell how investigators followed an astonishing trail from France to Surrey via Switzerland, Iraq and Louisiana, USA, with multiple arrests, unbelievable theories, false leads and dead-ends.
Ten years on, questions are raised about the effectiveness of the investigation and the impact of the media storm on the loved ones left in the wake of the murders. Ex cop Mark Preston shares his theory that the investigation got off on the wrong track from the start and looks at how the case could yet be solved.
All three episodes of Murder in the Alps will be available to stream or download for free on All 4 following the transmission of this episode on Sunday 26th June.