KINGSTON, Ont. - Investigators were piecing together Wednesday the shocking mystery of how three teenage sisters and one of their relatives ended up dead in a car in the Rideau Canal, northeast of Kingston.

The bodies of three teens, ages 13, 17 and 19, all from Quebec, were found along with a 50-year-old woman, described only as a relative, inside a vehicle that had submerged in the northernmost lock at Kingston Mills.

The group had been on vacation in southern Ontario and were on their way back to Quebec, said police in Kingston, about 260 kilometres east of Toronto.

"It breaks my heart just saying it," Const. Michael Menor said in an interview with The Canadian Press.

"You have a family, they've all drowned. It's just horrifyingly tragic."

Built in the early 1800s, the giant locks are used to lift boats between different levels of water. Investigators are puzzled, Menor said, because they have yet to determine what the car carrying the four Quebec women was doing in the area.

"It's very, very peculiar. There's no rhyme or reason," Menor said.

"It's just not an area that anyone would drive their car out to."

Kingston Police believe the car ended up in the water sometime Monday overnight. Parks Canada employees called police around 10:30 a.m. Tuesday after discovering the vehicle, submerged in about five metres of water.

Later that day, police divers pulled the bodies of the four victims along with their car out of the water.

So far, police say no one has come forward to say they saw the car go into the water.

"Again, it's strange," Menor said.

Police declined to release the names of the victims or their hometowns, saying loved-ones had not been notified. A national news agency, however, reported that all four victims were from Montreal.

The case is a coroner's investigation, but police haven't ruled anything out. Menor said police consider the deaths suspicious until the investigation proves otherwise, something that will be aided by autopsies scheduled for Thursday in Ottawa.

As part of the probe, police have seized the car and turned it over to mechanics for an inspection to see whether there was a mechanical breakdown or other forensic evidence that might help investigators.

"Right now we've got pretty much our whole detective office trying to figure this one out. Hopefully we can," Menor said.

"It's a bit of a mystery, but we're going to do our darndest."

Several others across the country also mourned family members killed on the Canada Day holiday.

Investigators were probing an early morning crash that killed one man on a stretch of the Trans-Canada Highway running through the Sudbury, Ont.

In Calgary, a 51-year-old man died after his Pontiac Grand Prix collided with the back of a tow truck removing a broken down vehicle at the side of the around just after midnight.

Another man, 24, from Mill River, N.S., was killed after his car overturned in a deep ditch and ejected his body in a single-vehicle collision just before 6 a.m. in Brooklyn, N.S.