Did violent video game Call of Duty spark gun-crazed loner's killing spree? Adam Lanza 'spent hours with game just like Anders Breivik'

The Call of Duty games have been linked to other mass killings including the Norwegian massacre by Anders Breivik and murders at a Jewish school in Toulouse by Mohammed Merah.

Breivik said he spent up to 16 hours a day playing the game in preparation for the attack.

He said he became a deadly marksman by honing his gun skills on the Modern Warfare part of the series.

Merah's wife, Hizia Miriam, said in June that she and her ex-husband would play Call of Duty together before he gunned down seven people in cold blood.

Violent video games can alter the brain in just one week and make players more aggressive, researchers say.

An Indiana University School of Medicine study found key areas in the brain suffer reduced activity and leave it physically altered.

The findings fuelled the debate over the impact violent games have on regular players and links to anti-social behaviour.