Toronto Police interviewed Bruce McArthur in 2013, in connection with three missing men who vanished from Toronto's Gay Village, more than four years before he was arrested and charged with six counts of murder.

A source with knowledge of the investigation told The Globe and Mail that police had linked Mr. McArthur to two of those three missing men through his dating apps. An anonymous tip from 2013 then led police to interview him.

Mr. McArthur was, however, not arrested until Jan. 18, after four other men were alleged to have been murdered.

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The Globe first contacted the Toronto Police Service about the new information on Sunday but the service declined multiple requests for comment. Homicide Detective Sergeant Hank Idsinga said on Sunday that the force "will not be commenting on who we may or may not have spoken to."

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Toronto Police spokeswoman Meaghan Gray declined comment again Monday.

On Tuesday, the Toronto Star reported that Mr. McArthur was brought in for questioning by police on a separate incident unrelated to men missing from Toronto's gay village. The newspaper said that homicide detectives did not find out about this previous interview until after McArthur's arrest . Ms. Gray confirmed to the Toronto Star that this information has been passed to the Toronto police professional standards unit and an internal investigation has been started.

Ms. Gray did not respond to The Globe's request for comment on Tuesday night.

The new information comes as the force is under pressure by the LGBTQ community to explain whether it treated the three initial missing-persons cases seriously enough at the time.

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The Canadian Press reported on Wednesday that Idsinga said that in the course of his work on the case he came across "concerning" information that has now triggered an internal police probe.

Det. Sgt. Hank Idsinga wouldn't provide further details, but said he became aware of the information after reviewing two previous police investigations into five missing men from Toronto's gay village.

"I saw something I felt needed to be investigated further," he said in an interview.

Last week, Idsinga prepared a report with his findings and sent it to the force's professional standards unit.

"I think you should take a look at this because we're accountable for what we do," he said he told the internal investigators.

"I'm not the one to decide whether mistakes are made or not, but I think it's something that certainly needs to be investigated. It was concerning."

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Two of the men investigated under Project Houston, Skandaraj Navaratnam and Majeed Kayhan, are among the six men that Mr. McArthur has been charged with murdering. No charges have been filed in connection with the disappearance of the third man, Abdulbasir Faizi, however police continue to investigate a connection to Mr. McArthur.

In an editorial board meeting with The Globe last week, Toronto Police Chief Mark Saunders said that he was open to "any kind of inquiry" into the service's handling of the missing men investigations, but stated that he believed his officers had done their best.

"If you don't have the evidence then you dig harder to make sure. You double check with all these investigations. You don't just sit back on your hands and `Nothing's come in, see you later.' We go out and aggressively try to pursue and look for that," the chief said. "We knew that people were missing and we knew we didn't have the right answers. But nobody was coming to us with anything."

The source who spoke to The Globe refused to be identified because they were not authorized to release information about the case. The source said the Houston probe began after Toronto police received a tip in November 2012 regarding Internet cannibalism allegations, which led them to look again at the disappearance of Mr. Navaratnam.

The 40-year-old was last seen on Sept. 6, 2010, leaving Zipperz, a bar in the Wellesley-Church village, with an unknown man. Mr. McArthur was also known to frequent Zipperz.

The tip about cannibalism led police to arrest a Peterborough resident, James Alex Brunton, who was charged with possession and importation of child pornography. However, the cannibalism was found to be fantasies and he was not responsible for Mr. Navaratnam's disappearance, the source said.

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After the link to Mr. Navaratnam proved unfounded, the task force's resources were significantly reduced, the source said.

However, while investigating, the source said police noticed similarities between Mr. Navaratnam and Mr. Faizi, 44, and Mr. Kayhan, 59, who had vanished, respectively, in December, 2010 and October, 2012.

In June 2013, eight months after Project Houston was started, police publicly linked the three missing men, launching a public campaign, asking for information on their disappearances. It was after that appeal that the tip identifying Mr. McArthur came in.

The source with knowledge of the investigation said police then interviewed Mr. McArthur, but it is unclear what further steps they took.

Two weeks ago, CTV's W5 program aired an interview with a friend of Mr. Kayhan who said he brought Mr. McArthur's identity to the police after his disappearance, telling the service that the last time he saw his friend alive he was with Mr. McArthur. "I didn't know Bruce's surname but I knew what he looked like and I knew what his username on the Internet was so I relayed that to the Toronto police," the man told CTV.

The Globe has not been able to contact the man, whose face was blurred and whose name was not broadcast.

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Toronto Police have told The Globe that 12 investigators were assigned to Project Houston but refused to provide a more detailed breakdown about the length of time they remained on the probe.

The police source who spoke to The Globe said that, once the cannibalism tip was vetted and thrown out, the team was reduced to two investigators, managed by Detective Debbie Harris of 51 Division. Now retired, she declined to comment on the investigation when previously contacted by The Globe.

Project Houston was disbanded in April 2014. A case summary prepared by the Toronto Police in 2016 reads that Project Houston conducted "over one hundred interviews with friends, family, neighbours and acquaintances" but that it ultimately failed to uncover what happened to the three men.

That year, investigator Joshua McKenzie said the "mammoth" investigation remained "open," but "suspended."

When asked in January, Det. Sgt. Idsinga said that Mr. McArthur was not a suspect during Project Houston, but offered a caveat to that designation.

"I really want to delineate between investigating a missing person and a homicide," Det. Sgt. Idsinga said. "Until we can establish that there has been foul play, we can attach a criminal offence to foul play, that's when someone would become a suspect."

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When asked to clarify, Det. Sgt. Idsinga offered: "We have to include people in our investigation and we have to exclude people in our investigation. That includes witnesses, persons of interest and suspects … In a missing-person occurrence, unless there's foul play involved that we have to be able to prove with evidence, people are not going to be assigned those particular terms yet."

Mr. McArthur eventually came again to the attention of police when they investigated the disappearance last summer of two other gay men, Selim Esen and Andrew Kinsman. Mr. McArthur had a relationship with Mr. Kinsman. Police seized his van after he had sold it to a scrapyard and found blood inside.

Investigators have since recovered the remains of seven men from garden planters at a Toronto home where Mr. McArthur worked as a landscaper.