"Why is it so… quiet?" Inside the nanny murder trial that is gripping the world.

Marina Krim looked around the ballet studio. Her daughter, Lucia, aged six, wasn’t there, nor was her two-year-old son, Leo. The New Yorker had arranged with their nanny to meet them at 5:30 that afternoon. Krim called the woman, and texted. “Where’s Lulu?” “Donde estas?” “Where are you?”

No answer.

Clutching her middle daughter’s hand, Krim arrived home to her luxury Upper West side apartment to find the lights off, the rooms within silent. Marina checked with the building’s doorman, but he hadn’t seen her children leave. As the pair rushed from room to room, Krim noticed a crack of light beneath the bathroom door.

“I’m like, ‘Oh God, it’s so quiet in here, oh God. Why is it so … quiet?’” Krim told the New York Supreme court, according to TIME. “And I open the door … And I open the door, oh God!”

The two children's throats had been cut, their bodies placed in the bathtub. Lucia, or 'Lulu', had also suffered 20-30 stab wounds.

After seeing her children's bodies, Krim noticed her nanny, Yoselyn Ortega, sitting on the bathroom floor. The Dominican woman, covered in blood, locked eyes with her employer for a moment before she raised a knife and began stabbing herself. Marina scooped her four-year-old into her arms, and the pair fled the apartment, screaming.

“It was a scream you can’t imagine is even inside of you,” she said, according to TIME. “I don’t even know where it came from. I just thought: 'I’m never going to be able to talk to them ever again. They are dead. I just saw my kids dead.'”

Krim's husband, Kevin, then a CNBC television executive, was returning from a business trip in those moments, according to CNN. When the father stepped off his flight at New York's John F. Kennedy International Airport, waiting police broke the tragic news. His children had been murdered, their nanny was in custody in hospital.

Ortega is currently facing trial for murdering the two children on that October 2012 afternoon - harrowing events that inspired the best-selling novel The Perfect Nanny.

The focus of the trial, which began in Manhattan on Friday, will not be to determine whether the 55-year-old committed the crime, but why she did.

According to US media, the mother of one, who worked for the Krims for two years, is pleading not guilty by reason of mental disease or defect. While she acknowledges she took the children's lives, she claims she was too mentally unwell at the time to understand the consequences.

This, her defence lawyer told the court, was because of a severe, undiagnosed mental illness she had suffered since childhood, including hallucinations and psychotic episodes.

According to The Cut, Ortega used prayer and religion to cope, "culminating in the moment when, after the murders, she told a psychiatrist she had felt the devil penetrate her body. She heard voices commanding her to kill."

In the years since, Kevin and Marina Krim have had two more children, whom the father described in a blog post as, "genetically and spiritually half Lulu and half Leo."

In memory of their children, the couple launched the Lulu & Leo Fund to support arts programs for children.

In Kevin Krim's words, it has become a legacy for their son and daughter distinct from the tragedy; "an act of positive defiance".