Alleged serial killer Bruce McArthur was under police surveillance when officers were forced to make a quick decision to enter his home and arrest him, believing a life could be in danger, police sources confirm to the Star.

On the morning of January 18, police watching McArthur observed a young man entering the Thorncliffe Park building where there the 66-year-old landscaper rented an apartment. When he walked in the 19th floor unit, they knew they had to intervene, according to a police source with knowledge of the events.

When officers decided they had to take action, they entered McArthur’s apartment and found a young man tied up but unharmed, according to the source.

None of the information about the circumstances of McArthur’s arrest has been officially released by Toronto police.

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Inside that same apartment over the weekend — 10 days since the arrest — forensic investigators were still collecting trace evidence, gathering clues at just one of the more than two dozen addresses where McArthur may have left evidence of his alleged crimes.

McArthur, now accused of five counts of first-degree murder allegedly spanning from 2012 to last year, is now the subject of what’s poised to become the most ambitious homicide investigation in the city’s history.

“This will end up being, including in terms of the potential scenes, the largest (investigation) Toronto has undertaken,” said Mark Mendelson, a former Toronto police homicide detective, in an interview Tuesday.

Nearly two weeks after announcing McArthur was charged in the deaths of Selim Esen, 44, and Andrew Kinsman, 49, Toronto police Det. Sgt. Hank Idsinga announced Monday that three more first-degree murder charges had been laid against him.

Homicide Detective Sgt. Hank Idsinga updated the media on the case against Bruce McArthur on January 29. Toronto Police have charged McArthur with three additional counts of first-degree murder, bringing the total number of charges to five. Idsinga also said that McArthur is an "alleged serial killer." (Toronto Police Services/YouTube)

Investigators now believe that Majeed Kayhan, 58, Dean Lisowick, 47, and Soroush Mahmudi, 50, were killed by McArthur, who was known in Toronto’s Gay Village.

Police have not released any of the causes of death, though Idsinga has previously said they have a pretty good idea of how Kinsman and Esen died. Investigators have also said little about how McArthur came to know each of his alleged victims, though they have confirmed he and Kinsman were in a relationship.

Idsinga told reporters Monday that the ongoing probe — which includes 30 properties to which McArthur is linked through his landscaping work — is “unprecedented.” Police located the dismembered remains of three unidentified people inside large planters at a home in Leaside.

A team of dedicated investigators, forensic specialists and dozens of officers, including some from the K9 unit, have been scouring the properties, with the belief that yet more human remains could be found.

“I have seen large scale investigations before with dozens and dozens of officers working on them. We’ve never seen anything quite like this,” Idsinga said.

The technicalities and complications in this case are mind-boggling, Mendelson said. Among the most challenging tasks will be the identification of human remains.

Mendelson said investigators may rely on dental records to trace remains back to an identity, though Idsinga said what has been found so far are skeletal remains.

Police may also try to track DNA through “reverse paternities” with family members, a technique that requires DNA from a mother and a father, Mendelson said. But that tactic relies upon having an idea of who is missing in the first place, something that is far from guaranteed in this case.

Lisowick, one of the three men who McArthur was charged with murdering Monday, was in fact never reported missing. Police have not revealed how they were able to obtain sufficient evidence in his death to charge McArthur.

Idsinga acknowledged Monday that a complication in the case is that it’s possible that there are more victims who not have been reported missing at all, or may have been reported missing in another place entirely.

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“If anyone comes into Toronto for an event such as Pride and then get reported missing back in their hometowns, the complainants not being aware that they were in Toronto — that’s obviously something we have to look at.

“As we sort out these remains and get them identified, we may find more people who were never reported missing,” Idsinga said.

McArthur is due back in court February 14.

Wendy Gillis can be reached at wgillis@thestar.ca