Ontario police said four young people from the greater Toronto area have died in a plane crash in the province's southwest.

According to police, a single engine Cessna 172 crashed around 8:30 p.m. Friday in a cornfield in Mapleton Township, about 50 kilometres northwest of Kitchener.

All four were pronounced dead at the scene. One of the victims was a 19-year-old woman. The three others were in their 20s, according to police.

Emergency crews were notified about the crash at 8:39 p.m. ET, but OPP Const. Keith Robb said the plane went off the radar 19 minutes earlier. An emergency transponder signal had been activated.

It's unclear how long the plane had been in flight before it crashed.

At this point, the investigation remains in its preliminary stages.

"We don't believe weather was a factor," said Robb.

Two investigators from the Transportation Safety Board remain at the scene, and are trying to determine whether mechanical failure is at fault. The plane was expected to be removed from the field later Saturday.

"Right now we’re basically in a field phase of our investigation where we will collect all the data we can to determine where we go with this investigation from here," said Ken Webster, an investigator from the transportation board.

The single-engine plane went down in a corn field near Moorefield, about 50 kilometres northwest of Kitchener. (Courtesy Global Toronto)

Webster told CBC that the plane crashed at a high speed.

Llori Nicholls was walking the dog with her husband last night when they saw the plane "making some spirals." She said at first they wondered if the plane was doing tricks.

Nicholls told CBC News it soon became clear the plane was in trouble and she could hear the engine sputtering.

"The whole time the nose was pointed straight down," she said.

She and her husband stood praying and hoping that the pilot would be able to pull out of the dive.

"Unfortunately the engine gave completely out," she said. "It went dead silent and plummeted down. And within a second or two we heard a really loud bump and our hearts sank."

Nicholls said she's distraught over how scared the passengers must have been.

"There was definitely time in their descent for them to be terrified, and it just kind of doesn't sit good with me," said Nicholls, her voice quivering.

Curtis Bults was getting ready to leave for a baseball game when he heard the plane go down behind his house. "It sounded like a whiny noise, like a go-kart, like a small plane going 'Eee Eeee Eeeee!,' said the 21-year-old.

"And [then] a couple second delay, and I heard a thud. I heard kind of a shake in the ground."

Bults said his two dogs were "absolutely freaking" from the noise, which could be heard clearly even though all the windows in the house were closed.

He then jumped onto his ATV to investigate. He drove it through the adjacent cornfields but couldn't find anything. After about 10 minutes, he returned home.

Bultz said it took him, his neighbours and emergency crews until 10:30 p.m. before they were able to locate the plane wreckage.

Once it was found, his father used a tractor to carve out a path for the emergency crews to get to the scene, he said.

"It was the middle of nowhere," said Bultz. "It was in the middle of a 50-acre cornfield."

Plane was a rental

Bob Connors, the general manager of the Waterloo-Wellington Flight Centre, said the plane was a rental and the group was sightseeing. The flight plan indicated they were heading to Niagara Falls and then Toronto.

Connors said the flight school, which operates out of the Waterloo Region International Airport, had not had a crash like this in a "long, long, long time." He said the staff knew the victims.

Connors would not comment on the experience of the pilot.

The victims' bodies have been transported to a hospital in Hamilton for autopsies.

The transportation board has said it could take several days before it knows what caused the crash.