Police investigating last month’s mass shooting in the Canadian province of Nova Scotia have launched a psychological analysis of the deceased gunman, in the hope of better understanding what led him to kill 22 people over a 12-hour period. The Royal Canadian Mounted police said on Monday that they were carrying out a “psychological autopsy” on Gabriel Wortman, who carried out the worst shooting in the country’s history. “The RCMP’s Behavioural Analysis Unit is conducting a psychological autopsy of the gunman, with the intent of gathering insights into why he committed the acts of violence. This includes an analysis of his personality, past behaviour and how he related to others,” said the statement. The technique, which involves extensive in-depth interviews with friends, family and colleagues, has been used for nearly 70 years, often to determine motives behind suicides. But the practice can also provide clarity in cases of mass murder. “These are much more than police inquiries,” clinical and forensic psychologist Antoon Leenaars told the Canadian Press. “They are investigations of the mind.” Leenaars, who wrote the 2017 book, The Psychological Autopsy, said the aim of the investigation is to construct a robust and detailed portrait of an individual, with the hope of providing clues as to why they acted in a particular way. “Given what this was designed for, I think this would be a perfect situation … That evidence can be extremely helpful in terms of trying to understand what happened,” said Leenaars. The psychological autopsy is part of a sweeping police investigation, which includes analysis of 17 separate crime scenes. The RCMP said they used “ground penetrating radar” to scan the gunman’s property to determine if anything of importance was buried, but found nothing. Police also announced an update on the firearms – two semiautomatic handguns and two semiautomatic rifles – used by the gunman. “One of the guns has been traced to Canada. The remaining guns that were recovered are believed to have been obtained in the United States,” they said. After the shooting, the prime minister, Justin Trudeau, announced a ban on 1,500 models of “military grade assault weapons” last week – a move that has come under fire in recent days for loopholes that would allow gun owners to continue possessing the firearms. For much of his deadly spree, the gunman disguised himself as an RCMP officer, using parts of an authentic uniform and a replica police vehicle to evade capture and to lure victims. Police said on Monday that the decals used on the shooter’s vehicle were created by an individual at a decal company without permission from the owner. Both the individual who made the decal, and the owner, are cooperating with police. Police said on Monday they had spoken to 500 witnesses and are continuing to conduct interviews, asking anyone who had a “professional or personal” conflict with the gunman to come forward.