Canadian police have determined the cause of the tragic shooting rampage in the Province of Nova Scotland.

The gunman in Canada’s worst ever mass shooting began the weekend rampage that ultimately killed 22 people after his girlfriend escaped him when he assaulted her, a top police official said on Friday, Reuters reported.

Police did not give details about the nature of the assault or how she escaped and fled into the woods. Local media said she had been tied up. The woman’s name was not provided. She is currently recovering from her injuries and cooperating with police.

“It was a significant assault, and this individual female did manage to escape, and that could very well have been the catalyst to start the chain of events,” Nova Scotia Royal Canadian Mounted Police (RCMP) Superintendent Darren Campbell said in a news conference. “However, we’re open to all possibilities and we’re not excluding the possibility that there was any premeditated planning also involved.”

After the woman fled on Saturday night, she spent the night in hiding in the thick woods that surround the rural hamlet of Portapique, Nova Scotia, and only emerged on Sunday morning to call the 911 emergency number at about 6:30 a.m.

By then the gunman, 51-year-old Gabriel Wortman, had killed 13 people in Portapique. Disguised in a police uniform and driving a fake police car, Wortman went on to kill nine more people on Sunday morning.

Canadian Prime Minister Justin Trudeau, speaking at a daily news conference, urged Canadians to join a virtual vigil at 6 p.m. ET (2200 GMT).