KINGSTON, Ont. - A Montreal woman accused of killing her three daughters slowly changed her tune during a six-hour interrogation, as the investigator tried to appeal to motherly guilt, but he suggested her new story was just as implausible, court heard Wednesday.

Tooba Mohammad Yahya, 41, her husband Mohammad Shafia, 58, and their son, Hamed Mohammad Shafia, 20, are all charged with four counts of first-degree murder in the deaths of three Shafia sisters and Shafia's other wife in a polygamous marriage. They have pleaded not guilty.

Three teenage Shafia sisters, Zainab, 19, Sahar, 17, and Geeti, 13, along with Rona Amir Mohammad, 50, were found dead inside a car submerged in the Rideau Canal in Kingston, Ont., on June 30, 2009.

The family was on their way back to Montreal from a trip to Niagara Falls, Ont., and checked into a motel in Kingston in the early morning hours of June 30 after being on the road since 6 p.m.

The three family members were interviewed by police that day, and in videos shown to the jury, they can be seen suggesting that Zainab, who didn't have a driver's licence, took the car keys so she could take the Nissan Sentra for a joy ride. The car ending up in the canal was a terrible accident, they suggested.

But after investigating for several weeks, including using wiretaps, police came to believe that the family used their Lexus SUV to push the Nissan into the canal with the girls and women inside, court has heard. The Crown alleges the family believed the girls were dishonouring them by having boyfriends.

Yahya, Shafia and Hamed were arrested July 22, 2009, and on Wednesday the jury watched much of the video of the long interrogation of Tooba. RCMP Insp. Shahin Mehdizadeh was brought in to conduct the interview in Farsi.

Before Mehdizadeh enters the room, Yahya breaks down into tears and sobs for several minutes when she looks at pictures of her children. In court Shafia cried, a tissue to his face and his shoulders shaking, as he watched his wife weep on the video.

For much of the interview, Mehdizadeh goes over Yahya's story with her piece by piece, pointing out evidence to the contrary and accusing her of lying to him.

"This is not an accident, madam," he says. "You know what has happened here ... I don't want the lies. I don't want the lie that Zainab came and took the key and after that I didn't see her."

"Someone became God that night and decided that these three girls and this lady should have been killed," Mehdizadeh says. "When they did this, they didn't do their work right, I am telling you."

Eventually, Yahya starts changing her story, bit by bit, but begins by asking that Mehdizadeh not tell her husband.

She admits the three of them were at the canal that night, and that the four dead people never made it to the motel. Yahya tries to pin it on Shafia, saying she and Hamed were walking by the canal and they heard a splash, ran over and saw the car in the water. It was at that point that she fainted, she said.

"I grab my hair and fell down," she says. "When I got to the motel I was still not thinking that the girls had been fallen into the water. I thought that their dad had already taken them. I was thinking like this. Do you understand?"

Mehdizadeh tells her that is "impossible." Police know that Hamed didn't stay at the motel in Kingston that night with the rest of his surviving family, rather that he drove through the night to Montreal, where he hit a pole in a parking lot, Mehdizadeh says. He called police the morning of June 30 to report that collision, which the Crown alleges he staged to mask damage the Lexus sustained while pushing the Nissan into the canal.

Mehdizadeh is incredulous that in Yahya's version of events, Hamed didn't call 911 after seeing four people plunge into the water in a car, but immediately called police to report a minor, single-vehicle collision.

"Hamed put you in the motel and he went to Montreal," Mehdizadeh says. "Four women have died here, yet nobody calls the police to come and save them when this had happened. That's why I am saying that you are lying."

At several points in the interview Mehdizadeh appeals to Yahya to do her "motherly job" and tell the truth and do the right thing now for her dead daughters.

"I want you, for the sake of your own daughters, for the sake of your daughters' souls, if you have any respect for them, tell me the truth," he says.

Mehdizadeh adds that he is only 42, but his hair has turned grey by these sorts of terrible cases, by seeing pictures likes the ones of her children.

"You are still young," Yahya replies.