KINGSTON, Ont. - Three teenage sisters and their father's first wife were already dead when the car their bodies were in plunged into a canal, a prosecutor suggested in court Friday in the clearest indication yet of the Crown's theory in an alleged case of four so-called honour killings.

The plan went awry when the Shafia family's Nissan got hung up on the concrete edge of the canal, with four bodies inside, so they had to use the family's Lexus SUV to nudge it into the water, breaking the headlight and scattering pieces on the ground, Crown attorney Gerard Laarhuis said in court.

Those headlight pieces were one of the factors that first caused police to suspect the deaths were not an accident, court has heard.

Tooba Yahya, 42, her husband Mohammad Shafia, 58, and their son Hamed, 21, have each pleaded not guilty to four counts of first-degree murder. They're accused of killing sisters Zainab, 19, Sahar, 17, and Geeti, 13, along with Rona Amir Mohammad, 52, Shafia's first wife in a polygamous marriage, over family honour.

The bodies were found June 30, 2009, in a car at the bottom of a canal in Kingston, Ont., where the Montreal family stopped for the night on their way back from Niagara Falls, Ont.

Yahya began testifying in her own defence Monday, and spent the entire week on the stand, most of it under pointed cross-examination by Laarhuis. He said as court ended Friday afternoon that he had another two hours of questions for her on Monday.

When Laarhuis began putting to Yahya step by step what the Crown alleges happened that night, Yahya's demeanour visibly changed, she shrunk in the witness box, denying his story by simply repeating "no, never," until he described the Lexus pushing the Nissan in, and Hamed picking up broken headlight pieces, but missing a critical few.

"No, sir," she said through an interpreter as she straightened up, her voice cracking.

"We are not murderers. We were a very sincere and collected family. This crime, we will never do such a crime. Don't ever tell me such a thing. I am a mother. If you are a mother then you could have known that what's the heart of a mother for a child ... Don't ever tell me that I killed my children, never."

Either Yahya, Shafia or Hamed positioned the Nissan with "the bodies" inside at the locks and reached through the open driver's window to put it in first gear so it would go in under its own power, Laarhuis said.

He pointed to something "interesting" Yahya said in her post-arrest interrogation, when she had said the three were at the scene that night, but that she was unwell and didn't know what happened.

"If I was awake and they were pressing and putting them into the water I might have known it," she says in the July 22, 2009, interview. "As a human I would have been shaken or heard a sound of splashing or something."

The idea that someone was "pressing" the four women and girls into the water was raised first by Yahya in the interview, not the police officer, Laarhuis said.

"You said that because that's what you remember from that night, that somebody was pressing and putting them into the water," he said.

Yahya said she meant that the officer had told her someone who lived nearby heard "the sound of a splash," but Laarhuis said a splash is what happens when a car falls into the water, but splashing "is what people do when they're in the water."

The family maintains that the night in question they were getting tired driving from Niagara Falls to Montreal, having left Niagara at 8 p.m., so they stopped for the night in Kingston around 1:30 or 2 a.m. Soon after they got to the motel, Zainab went to her parents' room to borrow the car keys to get her belongings out of the car, and that was the last time the family saw her or any of the other deceased. Zainab must have taken them all on a joy ride gone wrong, the family says.

But the Crown alleges the three sisters and the woman who was like their mother never made it to the motel that night. Shafia and Hamed checked into the motel, dropped their surviving children off there, then drove back to Kingston Mills locks, where Yahya was waiting in the Nissan with the four soon-to-be-deceased, Laarhuis alleged Friday.

"The plan was that someone would drive (the Nissan) up to the canal wall, you needed a place where the car would drop straight down and be under the water," he put to Yahya.

"It couldn't be a place where the car would get caught driving into a lake or something like that. It had to fall and go right under the water, and that in part was why you left the (minivan) at home and bought a new car just one day before this trip. You wanted a car that was lower and cheaper."

Yahya disagreed.

Either Yahya, Hamed or Shafia took the Nissan with the bodies inside and positioned it in front of the locks where the car would be found the next morning, Laarhuis said.

"(One of you) reached through the open window, put the car from neutral into gear number one, thinking that on its own power the Nissan would go into the water," he said to Yahya.

"What none of you expected, what was not part of the plan was that the Nissan would get hung up."

Yahya disagreed.

The Nissan was then hanging on the ledge, bodies inside, according to Laarhuis. So one of the three accused drove the Lexus behind the Nissan and nudged it in, causing the damage found to both the back of the Nissan and the front of the Lexus, and causing the S and the E from Nissan Sentra to fall right beside the locks, where they were found by police in the morning, Laarhuis said.

The plan was always to go to police the next morning and report the four people missing, but they couldn't very well do that in the damaged Lexus, so Hamed drove through the night back to Montreal to return in the undamaged family minivan, Laarhuis said.

While in Montreal, Hamed drove into a post in a grocery store parking lot to stage the damage to the Lexus, court has heard.