The photo - a headshot of a bearded middle-aged man with his eyes partially shut - was released at a news conference where the lead detective announced police had found the remains of a seventh individual linked to McArthur.
"I did not want to release this picture and I am doing so as a last resort", said Det.
In a press conference Monday, Toronto Police Detective Hank Idsinga released a photograph of an unidentified deceased male and asked for anyone with information about his identity to come forward. It's unknown whether the remains belonged to the man in the photo.
At least seven people's remains have been found inside large planters on properties that McArthur had worked at as a landscaper.
Investigators didn't initially realize they had seven sets of remains in their possession, he said.
McArthur was charged with the first of six first-degree murder counts, in the case of Navaratnam, on February 23.
Dr. Michael Pollanen, Chief Forensic Pathologist for the Province of Ontario, said the causes of death are pending further studies. But so far only 3 people have been identified from the remains found, leaving 4 sets of remains unidentified, a extensive process in this case.
McArthur is charged in their deaths, as well as the presumed deaths of Selim Esen, 44, Majeed Kayhan, 58, and Dean Lisowick, either 43 or 44. They believe he met the men in the village and on gay dating apps for older men.More news: Upstate New York college on lockdown after 'real' and 'credible' threat
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It is believed that McArthur potentially kept photographs of his victims as "trophies", reports The Toronto Star. The remains are of Skandaraj Navaratnam, 40, Soroush Mahmudi, 50 and Andrew Kinsman, 49. Police have seized more than 20 planters as part of the investigation.
Bruce McArthur, 66, was arrested in January following an investigation into the disappearances of two men from a predominantly gay neighbourhood in downtown Toronto past year, which led police to the planters at a property that he used for storage.
MacArthur, 66-year-old landscaper, was charged with six murders in the first degree: according to police, it has a direct relation to the death of Skandarajah "Skanda" Navaratnam, Andrew Kinsmana, Selim Esen, Majid Kahana, Arusha Mahmoudi and Dean Lisovyk.
For years, members of gay community have urged the police to look closely at disappearances of men fearing there was a connection.
At the time, police said they had no evidence of foul play in the Project Houston investigation, which also looked into the disappearances of Kayhan and Abdulbasir Faizi, 44, whose case remains unsolved.
McArthur is due back in court on March 14.
Police believe that the photo was taken after death and have altered it to remove "artifacts" and to enhance it.
He is now being held at the Toronto South Detention Centre in suburban Etobicoke.