Marina left Lulu and Leo at home with their family nanny. Hours later, she made a gruesome discovery.

Content warning: This post contains details some readers may find distressing.

Young Lulu and Leo Krim were living an idyllic family life in the wealthy Manhattan neighbourhood, the Upper West Side.

Their mum Marina was a stay-at-home mum. Their dad Kevin, the Senior Vice President of CNBC Digital. Six-year-old Lulu and her two-year-old brother wanted for nothing. They lived in a comfortable Upper West Side apartment. Lulu loved ballet.

On October 25, 2012, Marina left Lulu and Leo at home in the care of their trusted family nanny of two years for a few hours while she took their three-year-old sister Nessie to her swimming lesson.

She never saw their smiles again.

A then 36-year-old Marina returned to their apartment after hearing Lulu hadn't arrived at her ballet lessons. She pushed open the bathroom door to find her children's small bodies lying in the bathtub covered in blood before watching their nanny attempt to take her own life, News.com.au reports.

Both children had been stabbed to death. Leo died from a single laceration to his throat. Lulu, from several wounds consistent with the young girl having put up a fight.

Marina took their sister out into the hall of their apartment building and dialled 911.

Through sobs, she made a fist, demonstrating a stabbing motion towards her chest to police.

"I just saw my kids dead," the mother screamed. She banged her head on nearby marble pillars as hard as she could, trying to "wake up from this nightmare."

***

Yoselyn Ortega opened her eyes. She felt a breathing tube down her throat, her wrists in handcuffs shackled to her hospital bed.

She had no memory of allegedly murdering Lulu and Leo two days earlier.

"I had to do everything and take care of the kids,” the former Upper West Side nanny communicated to the on-duty police officer NYPD Sgt. Yoel Hidalgo as soon as she was able, the New York Post reports.

She also spoke of the 'skin injuries' she suffered from the cleaning products Marina 'made' her use.

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"I worked as babysitter only and she wanted me to do everything so wanted five hours of cleaning every week."

Unable to speak due to her self-inflicted injuries, Yoselyn told Sgt. Hidalgo she'd been "possessed by the devil" and she could not remember killing the young children.

In an interview with The Daily News from her jail cell, she said, "I didn’t do that. Someone else did."

***

Yoselyn went berserk in the kitchen of the flat she shared with her sister three days before the killings, the New York Times reports, throwing pots and pans before claiming she couldn't remember doing so.

According to ABC News, Court papers state later that day Yoselyn made an appointment to see Manhattan psychologist Dr Thomas Caffrey. They discussed her feelings of stress, anxiety and sense of failure because of her fractured relationship with her son.

It's unclear whether she received a formal medical diagnosis during that session - Yoselyn's first contact with any mental health professional or service in 30 years.

Early on the morning of the alleged murders, Yoselyn knocked on her neighbour's door and asked if she could come inside for breakfast.

"I can't be alone," she pleaded to a teenage girl in the apartment. "I'm afraid."

***

Sitting in the hospital, Nessie clinging to her chest, Marina had no idea how her life was meant to continue after this loss.

"I was asking over and over to anybody who would listen, ‘What am I supposed to do tomorrow? How do I do this? What do we do?’," PEOPLE reports she said in a rare public speech in 2014.

"People often told us ‘Time will heal’ – that seemed more than a little daunting. There was no obvious path for us, no manual with step-by-step directions about how to deal with a blow like this, no teacher telling us what to do."

She thought of how Nessie's face lit up when she would draw, collect and make things. Of how, even though she missed her siblings, she would "laugh and enjoy herself and make every heartbroken person in the room smile."

"I remember looking at our daughter Nessie in the darkest days and I so admired her ability to just live in the present, seeing the world just as one should," Marina said.

"To me, it seemed like she had all the answers."

In 2012, Marina and Kevin started the Lulu & Leo Fund, a charitable organisation that helps children and families foster creative confidence and build resilience through programs that provide meaningful engagement with the principles of creativity.

"As the parents of Lulu and Leo, we know that art and nature played a critical role in their short, beautiful lives and in the life of our surviving daughter, Nessie. We believe every child on the planet deserves the deep engagement with art, nature, and creativity that our children had," Marina and Kevin wrote of the Lulu&Leo Fund's core purpose.

"It is important to us that they are remembered as the sweet, creative, and fun-loving children that they were."

Marina, Kevin and Nessie have since welcomed two baby boys to their family - Felix in 2013 and Lunis in 2016.

"With each new baby, we feel a special and closer connection to Lulu and Leo, who are a constant presence in their siblings’ looks, gestures and voices," Kevin said.

A post shared by Lulu & Leo Fund (@lululeofund) on Feb 22, 2018 at 10:50pm PST

***

Five years after Lulu and Leo lost their lives, closing arguments in the "Killer Nanny" trial will begin on Monday.

Jurors heard 10 weeks of gruesome testimony against Yoselyn. They gasped and cried, some asked to be excused during the trial, News.com.au reports.

The defence argued an insanity defence, that the former nanny's mind was "diseased". That she is "profoundly mentally impaired" and "lacked the ability to make any decision" on the day of the killings.

Now, that is for the jury and the Manhattan Supreme Court to decide. But one thing is inexplicably clear.

A little boy and a little girl were taken from this world far too soon.

For more information on the Lulu & Leo Fund and to show your support, visit the Lulu & Leo Fund website and follow them on Facebook and Instagram.

If you or someone you know is experiencing depression or struggling with their mental health, call Lifeline on 13 11 14, Beyond Blue on 1800 224 636 or Kids Helpline on 1800 55 1800. If you are in immediate danger or at risk of self harm, call 000.