A married couple in their 50s died after their car plunged into Lake Ontario at a popular park west of Toronto, police said Wednesday as they investigated what led to the vehicle going into the water.

Halton regional police said the blue sedan crashed through a concrete barrier and went into the lake in LaSalle Park in Burlington, Ont., at about 8 p.m. on Tuesday, landing about 12 to 15 metres from the shore.

Officers weren't able to get the bodies of a 55-year-old man and a 51-year-old woman out of the vehicle until the whole car was lifted out of the lake several hours later, Const. Ryan Anderson said, noting that his police force doesn't have its own dive unit so they had to call for help from officers in neighbouring jurisdictions.

"A diver had to go down and attach the hooks to the car," he said, explaining that the vehicle was hoisted out of the water using heavy machinery.

Bystanders had initially tried to dive down to the car to get the occupants out but didn't succeed, he said.

"By all accounts it seems like everybody jumped into the water and did everything they could to try and get these people out," Anderson said. "But just with the way the vehicle was sinking and the pressure, it makes it very difficult to extract people from the vehicle."

He noted that the incident was particularly scary because witnesses initially thought there could be children in the car. The park is popular among families, as it has a playground and a splash pad for kids.

"We weren't exactly sure what we were going to find," Anderson said, adding that police don't believe anyone else was in the car.

Anderson said an autopsy was set for either Wednesday or Thursday to figure out whether the driver had a medical condition that led to the crash.

In the meantime, he said, officers from the Collision Reconstruction Unit are investigating. They're also considering the possibility that the crash was intentional, or that there was a mechanical issue with the car.

He said the incident was difficult to deal with, but the police service is thankful for the efforts of everyone involved.

"It was certainly a tragic and tough situation for everybody there last night," he said. "We'd like to thank the witnesses, the other services that helped us, our officers, EMS and fire."