A couple prays next to a large wooden cross while visiting a memorial set up to honour the victims of the shooting at Sandy Hook Elementary School.

London - Scientists have been told to study the DNA of US school gunman Adam Lanza to see whether he had an genetic abnormality that left him with an unusual tendency towards violence.

Earlier this month Lanza shot dead 20 children and six adults at Sandy Hook Elementary School in Connecticut, before killing himself. The 20-year-old had earlier murdered his mother.

Police have so far failed to uncover a motive for the massacre. Geneticists at the University of Connecticut said they are trying to detect a mutation that could increase the risk of aggressive or violent behaviour, in a move believed to be the first of its kind for a mass-murderer.

The research was ordered by the Connecticut medical examiner who carried out post mortems on all the victims. - Daily Mail