Police in Kingston, Ont., say they still don't understand how a car carrying a 50-year-old woman and her three teenaged nieces ended up in the Rideau Canal. An autopsy might solve some of the mystery, but it might not explain how the vehicle passed through a series of barriers.

The family, which hasn't yet been identified, were on a vacation and were driving home to Montreal.

Their black 2004 Nissan Sentra was spotted by a Parks Canada employee on Tuesday morning, about 10 metres north from the doors of the canal lock northeast of Kingston. The car was submerged in about five metres of water.

There are no obvious tire tracks that explain what happened.

In order to get into this particular lock in the canal system, the car would have had to pass through at least two roadblocks, including a pair of poles on the dock.

John Bruce, a lock operator for Parks Canada, told CTV News, "It would be very, very hard to put it in there."

At first, Bruce thought it was a stolen car that had been dumped. "We didn't know until after that it was more serious."

On Thursday, the coroner's office said it expects to have a preliminary report completed by the end of the day, which should indicate whether the victims suffered any obvious signs of trauma prior to their deaths. But it will take longer to determine if drowning was the cause of death.

Found inside the vehicle were the bodies of three teenaged sisters -- ages 13, 17 and 19 -- as well as a woman described by police as a relative.

Police believe that the car went into the water sometime between midnight and 6 a.m. Tuesday.

But officers aren't sure what the family members were doing near the locks, in the location where their car was found.

Police divers also discovered that a car window was left open, and they aren't sure if the car floated downstream any distance.

"We have to really get into the forensics before we can really piece together how the vehicle went in and what movement the vehicle may have taken once it entered the water," Staff Sgt. Chris Scott said Wednesday.

Scott said the "tragic occurrence" is not an easy case to investigate.

"Anytime we have a multiple death scene, it's traumatic for the families and it's difficult for police and taxing on police resources because there's a lot of things we need to close off just so we can do the right thing for the victims and satisfy the family's needs," he said.

On Thursday, Const. Michael Menor told CTV.ca that the car is now at Kingston police headquarters and will be examined thoroughly by staff mechanics.

"It will undergo a complete mechanical safety check and everything, and we're looking for forensic information as well," he told CTV.ca.

While Montreal police confirmed that they were previously asked by Kingston police to locate a second vehicle in Montreal's Saint-Leonard area, Menor said investigators are currently "not looking for another car."

With files from CTV Ottawa's Jonathan Rotondo and Joanne Schnurr, CTV Montreal's Annie DeMelt and The Canadian Press