KINGSTON, Ont. - A Montreal family accused of drowning four people, including three teenage sisters, was thought to be leaving for the airport when they were arrested in 2009, having learned a day earlier that police suspected them of murder, court heard Tuesday.

The bodies of sisters, Zainab, 19, Sahar, 17, and Geeti Shafia, 13, along with their father's first wife in a polygamous marriage, Rona Amir Mohammad, 50, were found inside a submerged car June 30, 2009, in the locks in the Rideau Canal.

Almost immediately after a police officer began interviewing the surviving Shafia family members that day, he suspected the father, mother and the girls' brother knew more than they were telling him, he told court.

Tooba Mohammad Yahya, 41, her husband Mohammad Shafia, 58, and their son, Hamed Mohammad Shafia, 20, have each pleaded not guilty to four counts of first-degree murder.

Their trial heard Tuesday that police had been building their case against the family, right from the days after the deaths up to the day before the arrests, when police executed a search warrant of the family home. The warrant made clear to the family that police believed they had killed their four family members, Det.-Const. Geoff Dempster testified.

Police also installed listening devices in the home during the search warrant to record what Shafia, Yahya and their son said after the search was over, Dempster said. The wiretap has not yet been played for the jury. The plan was to allow for one day to gather that evidence, then arrest the three on July 22, 2009.

In the early morning hours of July 22, a surveillance team saw the three accused get into their minivan and leave the house, and thought they were heading to the airport, so they were arrested then, Dempster said.

Dempster is the officer who conducted the first interviews with the family on June 30, 2009. In videos of the interviews with Yahya and Hamed, played for the jury Tuesday, Dempster becomes suspicious, particularly with Hamed. The jury watched Dempster's interview with an emotionless Shafia on Friday.

The family had stopped in Kingston, Ont., on their way home from a trip to Niagara Falls and they all told police Zainab took the car keys to get her clothes from the car after they stopped at a motel around 1:30 a.m. that night, and that was the last they saw her. They all hinted that although she didn't have a licence, she was eager to learn to drive.

But in the videotaped interview with Hamed, Dempster clearly has a difficult time imagining a scenario under which the four women and girls would go for a joy ride together at 2 a.m., when they had been on the road non-stop since 6 p.m. that day. Dempster also questions Hamed extensively about why he drove through the middle of the night back to Montreal instead of staying at the motel that night with the rest of the family.

Dempster suggests Hamed didn't take a direct route back to Montreal, but maybe witnessed something and isn't being truthful. He confronts Hamed with the information that someone on a boat near the locks the night before heard a loud splash and also saw another large vehicle drive away.

"You mean that someone pushed them in?" Hamed asks.

Dempster says that's not what he was implying, but now, at the murder trial, that's exactly what the Crown alleges. The family is alleged to have used their Lexus SUV to push their Nissan Sentra, with the four victims inside, into the canal in the middle of the night.

The Nissan's back end was damaged, as was the front end of the Lexus, court has heard. The Crown alleges Hamed staged a collision with a pole in Montreal with the Lexus to account for the damage.

Dempster interviewed Hamed for a second time that evening after finding out Hamed had neglected to tell him about the collision in Montreal. Hamed appears at turns anxious and at turns bored in the interview.

"You know what, I'm telling you, I'm already in a lot of mess," Hamed says. "I don't know what you're doing, if you want to blame it on me. I don't know where you're going with this," he says.

Dempster excuses himself and leaves the room. While he is gone, Hamed is seen biting his nail, cracking his knuckles, reorganizing cards in his wallet, reading through Dempster's notes and flexing his biceps.

Police really began to suspect the family on July 3, Dempster testified, when they discovered similarities between pieces of plastic found on the ground at the locks in Kingston and the Shafias' SUV.