French police investigating the murder of a British family on a remote Alpine road two months ago have widened their probe to include mentally disturbed people after failing to find a motive.

Saad al-Hilli, an Iraqi-born British engineer, was found shot dead with his wife and mother-in-law in their car on a remote mountain road, with the body of a French cyclist, Sylvain Mollier, discovered nearby.

French prosecutor Eric Maillaud said: "We are looking at all mentally unbalanced people, very violent or known to be capable of violence, using weapons."