"If anyone knew before us, it's people who knew [McArthur] very, very well", Saunders told The Globe and Mail.
"I did not want to release this picture and I am doing so as a last resort", said Detective-Sergeant Hank Idsinga, lead investigator in the case.
After releasing the photo, Idsinga announced police have also found the dismembered remains of a seventh victim buried in planters at a home on Mallory Crescent where McArthur, 66, worked as a landscaper.
Idsinga warned the photo "may be disturbing" to some people, but he declined to say how the photo was taken.
He said they showed the picture to members of the gay community but could not identify him. The man is dark-skinned, black-haired, and bearded, and appears to be of African or Middle Eastern descent.
Police are not sure how many more victims will be discovered.
McArthur is now being held at the Toronto South Detention Centre in suburban Etobicoke.
Numerous other alleged victims frequented the "Gay Village" area of Toronto where police believe they met McArthur through gay dating apps, CBC reports.
When detectives burst into his Thorncliffe Park apartment, they quite possibly saved a young man's life.
Police stand guard at a house on Mallory Crescent in Toronto on Tuesday, January 30, 2018.
They also released a photograph of a man who they believe is a victim, saying they hoped releasing the image would help identify him.
Idsinga clarified that authorities are unsure if the unidentified man is related to the found remains.
The police does not exclude the possibility that some of the unidentified remains may belong to the Esen, Kahanu or Lisovyk.
He said they realize how hard it might be for a relative or friend to see the picture and realize the man is deceased.
"I've never done this before", he said about releasing the photo. "I do it with great hesitation".
"We have utilised numerous investigative techniques to identify this individual and so far have been unsuccessful". Only three sets of remains found in those planters have been identified and linked to Kinsman, Mahmudi and Navaratnam.
The 66-year-old self-employed landscaper is charged with first-degree murder in the deaths of six men who had disappeared from or had ties to Toronto's gay village.
The investigation, which police have described as unprecedented in scope, has proceeded on multiple fronts, with investigators gathering evidence, searching multiple properties and examining hundreds of missing persons cases in search of potential links to McArthur.
The cause of death of the victims had not yet been released by police.