A preliminary autopsy report is expected on the bodies of a Quebec family found in a car submerged in the Rideau Canal near Kingston.

Police retrieved the bodies of a 50-year-old woman and three teenaged sisters, ages 13, 17, and 19, on Tuesday. Investigators are treating the deaths as suspicious.

"This is an emotional incident," said Const. Michael Menor of the Kingston police. "What bothers me is the fact there's three sisters. It bothers us, so we have to work through that professionally."

The bodies were found in a black 2004 Nissan Sentra with Quebec plates. The car was submerged in about three metres of water at the first set of the Rideau Canal's lock doors near Kingston Mills Road, a rural road just off Highway 15 used mostly by local residents.

Police said determining how the vehicle got into the water is puzzling. Investigators haven't been able to locate any tire tracks near the water's edge and police say it's an unusual spot for a car to plummet into the canal.

If the car entered the water near the spot where it was found, the vehicle would have had to pass over rocks and a curb. It also would have had to get through a gate that was closed.

Police divers noticed a window in the car was left open, and it's currently unclear if the car could have floated downstream from another location.

Investigators believe the car ended up in the water sometime between midnight on Monday and 8 a.m. Tuesday.

"In the dark of the night, we're still asking why are they there? What happened? Our investigators are doing their darndest to get through this right now," said Menor.

The coroner's office in Ottawa said it's expecting to have a preliminary report on the bodies completed by the end of the day.

The report is expected to indicate whether the victims experienced any obvious signs of trauma prior to their deaths. The office said it will take longer to determine if drowning was the cause of death.

The vehicle was discovered by a Parks Canada employee who noticed an oil slick on the water Tuesday morning.

"I looked at it for quite a while to make sure I was seeing what was there. I thought it was a stolen car that had been dumped. We didn't know until after that it was more serious," said lock operator John Bruce.

The bodies were later found by a boater who dove into the water to take a closer look at the vehicle. The car is now being examined at Kingston police headquarters.

"Mechanics staff are doing a safety inspection on the vehicle and forensic staff are also examining the car. That might point us in a direction," said Menor.

Police said the group was on vacation near Toronto and was heading back to the Montreal area. The names of the victims have not been released. However, police said all four victims are related.

Anyone who may have spotted the black Nissan earlier this week is asked to call Kingston police at 613-549-4660.

With a report from CTV Ottawa's Joanne Schnurr