ST. JEROME - Courtroom spectators broke down sobbing as a grieving mother shared memories of her two children killed by her estranged husband, who was once a respected cardiologist.

About half the courtroom wept while listening to the testimony Thursday of Isabelle Gaston.

Even the killer Guy Turcotte, who had remained stoic throughout the proceedings, began weeping when he heard his wife's name.

She testified the couple always had its problems but she never believed Turcotte would hurt his own kids. She is one of the final Crown witnesses in its case against Turcotte.

Turcotte has pleaded not guilty to two counts of first-degree murder in the deaths of five-year-old Olivier and three-year-old Anne-Sophie in February 2009. He admits to killing the children but has denied intent.

The testimony was so highly anticipated that members of the public began lining up an hour-and-a-half before the courthouse opened, just to get a seat.

During nearly two hours of testimony, Gaston recalled the final moments with her children.

She dressed Olivier the morning of Feb. 20, 2009, and gave him a kiss as he went off to school. He waved back at her, the final time Gaston saw him.

A few hours later, Anne-Sophie would be dropped off at daycare. The mother scurried around the car for her daily exchange of 20 kisses with the toddler, who was sitting in the baby seat.

The jury heard that tensions had flared for an umpteenth and final time when Gaston had the locks changed at the home the couple once shared.

She switched them following a number of run-ins between her ex and her new boyfriend, Martin Huot.

She testified that Turcotte was livid.

"You want a war? You've got a war," she recalled him saying over the phone.

Gaston panicked at the threat. But she felt the warning was related to money.

The next day, the kids were found dead. She found out about it on the news.

"I feel stupid for not thinking that he'd hurt the kids," Gaston said. "I never thought for a second he'd do that."

Turcotte's gaze didn't meet hers as Gaston glared at him before beginning her testimony. But the minute the clerk said her name, Turcotte broke down and wept.

Gaston, an emergency-room physician at Hotel Dieu Hospital in St-Jerome, also cried as she told the jury about a relationship that began when they were medical students in 1999.

"From the beginning we had our highs and lows," Gaston said.

"In 10 years I couldn't say it was all black or all white."

The couple fought -- and fought a lot. But they had two children, Olivier in 2003 and Anne-Sophie in 2005.

They separated once, early on, after Gaston said she found gay porn on his computer.

That, coupled with a lack of intimacy in their marriage, would be the beginning of the end, she said.

Gaston would begin a new relationship with a personal trainer, Huot, which blossomed quickly but which she decided to keep secret.

"I regret today not having told Guy myself earlier," Gaston said, referring to how Turcotte heard the news from a third party -- Huot's ex.

"I waited because of Anne-Sophie and Olivier."

Gaston said she always considered him a good father. Even when the couple split, she had hopes they might still be friends. She said she wanted them to be "a team" -- to work together on raising the kids.

She last spoke to Turcotte on May 17, 2009.

She reached him at a psychiatric hospital in Montreal when an operator mistakenly transferred the call, allowing her to get through.

Gaston hadn't gone to work that day and was thinking of committing suicide herself. She had already written the letter. First, she says she wanted to ask Turcotte: why?

"Why the kids, Guy? I loved them more than I love myself," she told him.

He replied, according to Gaston's testimony: "Me too."