Toronto detectives probing the disappearance of one of the victims of serial killer Bruce McArthur initially investigated whether the missing man might have been targeted by another infamous murderer, Luka Magnotta.

The revelation is contained in a 185-page police affidavit, among nearly 6,000 pages of court papers unsealed Monday by Justice Cathy Mocha at the request of several media outlets, including The Globe and Mail.

The filings document how Toronto police tried to solve a series of disappearances from the city’s Gay Village between 2010 and 2017.

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Mr. McArthur was eventually unmasked and pleaded guilty in January to eight counts of first-degree murder. But years before he became a suspect, police investigated whether one of the missing men, Skandaraj Navaratnam, had been murdered by Alex Brunton, a self-proclaimed cannibal in Peterborough, Ont.

Project Houston, the police team investigating Mr. Brunton and the first three missing men, filed 41 requests for judicial authorizations to scrutinize his records, search his home and track his car. Another 44 requests were filed under Project Prism, the task force that eventually caught Mr. McArthur.

“The public has the right to know why extraordinary orders like search warrants are granted and in the matter before this court there is a substantial public interest in how the police conducted the investigation,” Justice Mocha said in her decision to release the documents.

The unsealed documents show that Toronto police deployed major resources under Project Houston to solve the disappearances of Mr. McArthur’s first three victims, but were not looking in the right direction.

Mr. Brunton was taken into custody in May 2013 after an elaborate six-month investigation that involved physical surveillance, online contacts, a bar-hopping night with undercover officers and a clandestine entry into his house.

When detectives realized that Mr. Brunton was not a murderer, they tried to check if he had a connection to Mr. Magnotta, who was in Montreal awaiting trial for killing Chinese student Lin Jun, dismembering him and mailing body parts. (Mr. Magnotta was later convicted of first-degree murder and four other charges.)

The affidavit says investigators noticed “circumstantial links” between Mr. Magnotta and Mr. Brunton.

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Mr. Brunton talked about being with another “cannibal” named Nathan that he had met at the Toronto male strip club Remington’s. Mr. Magnotta worked as a dancer there, from 2001 to 2003, and also used the pseudonym Nathan.

Also, between the ages of 16 and 18, Mr. Magnotta lived in Peterborough, minutes away from the Brunton residence. Lastly, Mr. Magnotta lived in Toronto at the time Mr. Navaratnam disappeared.

As a result, in September 2013, Toronto police applied for a warrant to retrieve data from Mr. Magnotta’s computers and cellphones.

Two months after seeking that judicial authorization, detectives interviewed the actual culprit, Mr. McArthur, who was known to be acquainted with at least two of the missing gay men. Prosecutors said at his sentencing that “McArthur was not a suspect at the time of this interview. Rather, he was treated as a witness.”

The documents don’t say what detectives found on Mr. Magnotta’s devices but Project Houston shut down the following year without locating the missing men.

Mr. Navaratnam, who turned out to be Mr. McArthur’s first murder victim, was last seen in public in Toronto in September 2010.

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In November 2012, Swiss police relayed a tip from an informant, Markus Dubach, who had been on a cannibalism fetish website called Zambianmeat.com. Swiss police considered him a reliable source who previously helped catch an actual serial-killing cannibal, Matej Curko.

Mr. Dubach said he had been in touch with a user called Chefmate50, who lived near Toronto, claimed to have eaten humans and talked about Mr. Navaratnam. Toronto police found that the Chefmate50 account belonged to Mr. Brunton, a 65-year-old retiree.

It later turned out that Mr. Brunton had never uttered Mr. Navaratnam’s name in his online chats. It was an inference made by Mr. Dubach. “The informant used the information provided to him by Brunton and did his own research,” the affidavits acknowledged.

Nevertheless, while investigating Mr. Brunton, police found that he exchanged nude photos with minors and chatted online about torture, killing and cannibalism.

Police kept him under surveillance and put a GPS tracker on his car. They covertly cloned the content of his computers. Undercover officers befriended him at hockey games and even took him to several clubs in Toronto, including Remington's.

At the end of April 2013, an undercover officer posing as a potential victim contacted him on Zambianmeat.com. Mr. Brunton instructed the officer “to come to Toronto to be killed,” the affidavit said.

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The Project Houston team planned an intricate take down right after Mr. Brunton was supposed to pick up the undercover officer at Pearson airport in Toronto.

Police installed on his car a remote-controlled speed limiter that would disable his ability to accelerate his Mitsubishi RVR. Officers also planned to break into his car while he was in the airport terminal to make sure it carried no weapons.

If Mr. Brunton didn’t show up at the airport on May 11, the backup plan was to arrest him at his home on May 13. The latter happened.

Officers showed up at Mr. Brunton’s home at 6:30 a.m. on May 13. He eventually pleaded guilty to several counts related to child pornography. But it would be another four years before Mr. McArthur’s arrest.