Four seniors who went missing in Muskoka two-decades ago are believed to be dead, police say.

The historical missing persons case is now a homicide case as provincial police investigate three properties in the Muskoka-area that housed the elderly people.

Joan Lawrence, who was 77 years old at the time, disappeared in 1998. Between 1997 and 1999, 91-year-old John Semple, 72-year-old John Crofts and 73-year-old Ralph Grant also disappeared.

OPP Interim Deputy Commissioner Paul Beesley said no human remains have ever been recovered, but Det.-Insp. Rob Matthews said all four are believed to be dead.

Matthews has been one of the lead investigators on the case since it began about 21 years ago.

Matthews said the retirement homes were owned by four siblings who have not co-operated with police during the probe. The owners are considered "of interest" in the case.

He said the two retirement homes and a farmhouse took in vulnerable seniors who no longer had contact with their families in the late '90s, and police regularly received complaints and concerns over how the occupants were treated.

Matthews said investigators learned residents would escape from the retirement homes in the night and go to homeless shelters in Toronto or Ottawa. He said 16 deaths occurred at the properties, but only 12 were reported to police.

He said the 12 reported deaths are not suspicious, but the four remaining deaths are now being treated as homicides.

Matthews said police were also investigating possible fraud regarding Lawrence's benefits, as well as concerns over how the property owners took payments from residents.

He said Lawrence, who was known at the time as "the cat lady," lived in a small shed on the farm property for two years with "too many cats to count" and that the owners charged her $600 a month for rent.

Matthews said he visited the property in 1998 after someone contacted police, then returned two days later with a health inspector. But he said by then, Lawrence was gone and the shed had been cleared out.

He said investigators now know Lawrence was moved about 600 metres to an abandoned van "where she met her end on that property."

Matthews said police are appealing for former employees of the retirement homes -- which are no longer operating -- and anyone with information on the case to come forward.

Anyone with new information can call a dedicated missing persons hotline toll-free at 1-877-934-6363 (1-877-9-FINDME) - in Canada only - or submit information by e-mail.

- With files from CTV Barrie