By COLLEEN LONG, Associated Press

NEW YORK (AP) — A defense lawyer says it was an act of madness. Prosecutors will begin trying to prove Thursday that it was murder.

Five years ago, Yoselyn Ortega, a trusted nanny to a well-to-do family, took two young children in her care into a bathroom at their Manhattan apartment, slaughtered them with a knife and then slit her own throat.

The killings of Leo Krim, 2, and Lucia Krim, 6, shocked New Yorkers, especially the thousands of parents who depend on loving nannies to take care of their children.

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At a trial that begins Thursday, the central mystery isn’t whether Ortega killed the children, but why she did it — and whether she was too mentally ill to be held responsible.

“This case is about the insanity defense,” defense attorney Valerie Van Leer-Greenberg told prospective jurors last week. “Not responsible by reason of mental disease or defect.”

Prosecutors plan to present their own psychiatric experts who will say that Ortega understood the consequences of her actions, even if her motive may never be understood.

“It’s the slaughter of two innocent children,” Assistant District Attorney Stuart Silberg said.

A judge has already declared Ortega, 55, mentally fit to stand trial.

New York has a high bar for the insanity defense and it is rarely successful. To win, Ortega’s lawyers will have to prove that she didn’t understand the consequences of her actions and didn’t know right from wrong at the time.

During jury selection, some potential members of the panel were dismissed after telling the court they believed mental illness does not excuse someone from a moral understanding of right and wrong.

Others were rejected as jurors after saying they couldn’t be impartial because they had small children themselves or a close relationship with a caregiver.

Ortega had worked for the Krims for about two years, in one of the city’s wealthiest neighborhoods, a block from Central Park, and by some measures had a close relationship with her employers.

Once, the family traveled together to the Dominican Republic to meet Ortega’s family.

Some of Ortega’s friends and relatives said she appeared to be struggling emotionally and financially before the killings, and she’d started seeing a psychologist.

A doctor who testified for the defense during pre-trial legal hearings said Ortega had serious delusions.

On Oct. 25, 2012, the day of the killings, she asked a neighbor to come over because she was afraid to be alone, Dr. Karen Rosenbaum said. And three days before, she was out of control at her sister’s apartment, throwing pots and pans, and then being unable to remember the outburst.

But police said that while she was in custody after the killings, Ortega gave interviews that paint a picture of an unhappy employee: She told authorities that she hurt the children because she was having money problems and was angry at the parents, according to court papers. She said her schedule constantly shifted, she had to act as a cleaning lady though she didn’t want to and she missed an appointment with “the psych,” according to the papers.

“Oh, my God, I’m sorry. I’m sorry for what I’ve done,” she said, according to the court papers. “Relieve me of my misery.”

The parents of the two slain children rarely speak publicly about the killings, but both are expected to testify.

Their mother, Marina Krim, discovered her two dead children and the bleeding nanny in the darkened apartment after returning from picking up another daughter, age 3, at a class.

Marina Krim and her husband Kevin Krim, a CNBC executive, use a Facebook page to post updates on how they are doing, writing about the arrival of two new children, Felix born in 2013 and Linus in 2016. Strangers post messages of support; more than 80,000 people like and follow the page.

In the wake of the tragedy, the couple started the Lulu and Leo Fund that aims to support innovative art programs for children and they recently posted a video message on Facebook asking that people mention the fund as their case becomes news again.

Featured Image: Lucia Krim, 6, and Leo Krim, 2 via Facebook