A criminologist and former police officer says Toronto police have their work cut out for them as they push further into their investigation of Bruce McArthur, a 66-year-old man accused of murdering two men who went missing in 2017.
Neither Andrew Kinsman's nor Selim Esen's bodies have been located, and police believe McArthur is responsible for the death of others. The charges against McArthur have not been proven in court.
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"Prosecuting one case without a body is problematic enough. Two, perhaps four, perhaps more? We're sort of in uncharted territory," criminologist and Western University professor Mike Arntfield told CBC Toronto.
Arntfield says in cases without bodies, police must rely on other evidence of death to build a case, often by locating blood.
They require "evidence of blood loss where the only inference to draw is that someone has bled to death and is now dead," he said.
Officers have spent days combing through the contents of McArthur's Thorncliffe Park apartment, as well as four other properties connected to him, three of which they've since released.
On Monday, an auto-shop owner who bought a minivan from McArthur last fall told CBC Toronto that the police had located traces of blood inside the vehicle.
Connecting victims a challenge
With charges laid in the cases of Kinsman and Esen, Det-Sgt. Hank Idsinga says police are now working to identify other possible victims.
Police haven't ruled out a possible connection between McArthur and three men who went missing in the Church and Wellesley area between 2010 and 2012. Police dubbed the investigation into their disappearances Project Houston.
On Tuesday, CBC News reported that one of them, Skandaraj Navaratnam, was romantically involved with McArthur.
Former Toronto homicide investigator Dave Perry told CBC Radio's Metro Morning that "serial killers are the most difficult investigations that a police service can face, that investigators can face."
"How do you recognize a serial killer?" he said. "Well, it takes several offences where the patterns start to form."
Perry, who is now the CEO of a private investigation firm, said cases like this one are extremely rare.
"We haven`t seen a case like this in quite some time," he said. "They only account for one per cent of our cases in Canada."
Killers often target marginalized groups
Also adding to the challenge in a large-scale murder investigation is the nature of the victims, who are often from marginalized communities.
"Serial offenders will generally target people who they're confident no one will be looking for," said Arntfield.
He said often the victims of serial killers are "perhaps not employed, perhaps new to the city," meaning their absence is less likely to be noticed.
Jooyoung Lee, an associate professor of sociology at the University of Toronto, agrees, pointing out that marginalized groups are also less likely to have a rapport with police.
"Serial killers target people who are easy victims, people who are in their neighbourhood, people who they may rub shoulders with at social events, and also people who are not likely to get a lot of attention from police investigations," he said.
Police have avoided calling McArthur a serial killer, saying that it's up to the media to decide whether or not to use the term.
A 2005 FBI symposium on serial murder defined it as "the unlawful killing of two or more victims by the same offender(s), in separate events."