French investigators believe the shooter behind the 2012 Alps murders is an ex-Swiss special forces agent who “lost the plot” after undergoing training in the use of “high-precision”, close-range gun techniques, say reports.

Mystery still shrouds the shooting of three members of a British-Iraqi family and a French cyclist on September 5 2012 in a quiet lay-by above Lake Annecy, in eastern France.

The victims included Surrey engineer Saad al-Hilli, 50, his wife, Iqbal al-Hilli, 47, and her mother, Suhaila al-Allaf, 74, as well as Sylvain Mollier, 45.

The al-Hilli’s seven-year-old daughter, Zainab al-Hilli, was shot in the shoulder and beaten around the head, while her sister, Zeena al-Hilli, four, escaped by hiding in the back of the family’s BMW car.

Detectives swiftly concluded that a Luger PO6, Parabellum 7.65, was used to carry out the murders, after pieces of the vintage Swiss Army weapon were found on the ground.