The authorities on the east coast of Canada were searching for a motive on Monday after a gunman who appeared to be dressed as a police officer and was driving a vehicle that appeared to be a Nova Scotia police car killed at least 16 people, one of the country’s worst mass killings in recent memory.
Gabriel Wortman, who ran a denture clinic in Nova Scotia, began the massacre in the town of Portapique on Saturday night and did not stop until he died 12 hours later at a gas station in Enfield, 22 miles away, the authorities said. The police have not said how he died.
Heather O’Brien, a nurse, was one of the victims of the shooting, and her daughter, Darcy Dobson, wrote in a Facebook post on Sunday that “a monster murdered my mother today.”
“She drove down the same street in the same town she drives through every single day,” Ms. Dobson wrote, saying that her mother had texted their family group at 9:59 a.m. Sunday. “By 10:15 she was gone,” she said.
A police officer and an elementary schoolteacher were also among the victims of the attack, which was described by Stephen McNeil, the premier of Nova Scotia, as “one of the most senseless acts of violence in our province’s history.”
In 2014, Mr. Wortman, a denturist, was in the news in Canada for a different reason: He was creating a new set of dentures, free of charge, for a cancer survivor who had lost all her teeth.
“My heart went out to her,” he told a Canadian TV network at the time. She had cried tears of happiness, speaking of the “angels” who came to her aid.
The motive for the mass shooting was not immediately clear. The police said that while the killings appeared to be targeted at the start, they became random as the rampage through Nova Scotia progressed.
“Our hearts are broken today as we attempt to accept the loss of my sister, Lisa McCully, who was one of the victims of the mass shooting in Portapique last night,” said Jenny Kierstead, whose sister, a schoolteacher, was one of the victims.
Another victim was Heidi Stevenson, an officer with the Royal Canadian Mounted Police with 23 years experience on the force and mother of two. She died after she responded to the shooting.
“Heidi answered the call of duty and lost her life while protecting those she served,” the police said in a statement. Another officer was injured.
Prime Minister Justin Trudeau called for unity in his public address on Sunday. “As a country, in moments like these, we come together to support one another,” he said. “Together we will mourn with the families of the victims, and help them get through this difficult time.”
Those who knew Mr. Wortman, 51, described him as having been “a little different,” but they were nonetheless shocked to hear that he had been identified by the authorities as the perpetrator of such a bloody attack.
“Gabriel always had a sadness about him, but I was so shocked to hear that he’d hurt other people,” Candy Palmater, a university friend, told local news outlets on Sunday. “I don’t know what his later adult life was like, but I can tell you that at university, people weren’t nice to him.”
She said she couldn’t reconcile what he had done with the man she knew.
Scott Balser, a former high school friend, said that Mr. Wortman “was a very nice guy who liked to help others,” according to The Chronicle Herald, a newspaper in Nova Scotia.
Sophie LeBlanc, a woman who wrote on Twitter that her mother went to high school with Mr. Wortman, posted a photo of his yearbook entry. “Gabe’s future may include being an R.C.M.P. officer,” it said.