The arrest of a man in connection with three decade-old murders comes as a relief to one former Toronto homicide department commander, who said detectives had been working on the investigation for 16 years.

"There is some relief: relief for the families – investigators over the years got very close to the families – and relief for the investigators," Gary Ellis told CTV's Canada AM on Friday.

"There were teams of investigators over the years who took this on and couldn't quite bring it home."

On Thursday, Peter Dale MacDonald, 52, was charged in the murders of three women found strangled west of Toronto's downtown between 1994 and 1997.

Police said the women were sex trade workers who were discovered near a pool on the shores of Lake Ontario.

MacDonald, originally from Prince Edward Island, is currently an inmate at Kingston Penitentiary for the 2000 murder of Toronto man James Campbell.

He was charged earlier this year in the 2000 death of Michelle Charette, in Windsor. Lawyer Kirk Monroe said MacDonald will plead not guilty when that case goes to court in December.

Homicide Det.-Sgt. Steve Ryan told reporters on Thursday that MacDonald had been charged with three counts of first-degree murder in connection to three cold cases.

Julieanne Middleton, 23, and Virginia Lee Coote, 33, were found dead in the troubled neighbourhood of Parkdale in 1994. Darlene MacNeill, 35, was found in the same area in 1997.

MacDonald was previously a person of interest in the women's death. Ellis said a break in the case about six months ago led to the charges.

"It is a great relief that Steve Ryan and his team could bring it all together," Ellis said.

Ellis is a 30-year police veteran who retired in 2007 at the rank of superintendent. He was previously the unit commander of the homicide squad and the sex crimes unit.

Ellis, now the program head of justice studies for the University of Guelph-Humber, said officers were still actively investigating the case when he joined the homicide department.

"It may have been forgotten by the public but it was never forgotten by the police. The boxes were in our office in 2003 when I was in homicide. The detectives were working on it then," Ellis said. "The pictures of the victims were on the wall; they were never forgotten."

None of the new allegations against MacDonald have been proven in a court of law.