Both prosecutors and defense attorneys agree: Yoselyn Ortega, a Manhattan nanny, repeatedly stabbed and killed the two young children in her care in 2012.
The question in court now is why -- and whether that matters.
Opening statements in Ortega's murder trial began Thursday in Manhattan Supreme Court, and Ortega stands accused of repeatedly stabbing to death Lucia, 6, and Leo Krim, 2, in their Upper West Side home in Manhattan.
In opening statements, prosecutors detailed the evidence surrounding the gory killings in which the two children were nearly decapitated and left to bleed in a bathtub. But prosecutor Courtney Groves told jurors they would not try to prove a motive in the case.
"You will be asking yourself why? Unfortunately, you may never have a satisfactory answer to that question," she said.
Rather than counter the evidence of Ortega's role in the killings, defense attorneys argued that she was suffering from depression, was hearing voices, disassociating from reality and experiencing hallucinations.
Ortega's lack of a motive is evidence, the defense argued, that she suffers from untreated severe mental illness and belongs in mental health care rather than behind prison bars.
It's been more than five years since Ortega was charged in the killings, and the Krims have waited as Ortega's attorney, Valerie Van Leer-Greenberg, tried to convince the court that her client was mentally unfit to stand trial.
After several competency hearings, Judge Gregory Carro ruled that Ortega was fit for trial.
Twelve jurors and six alternates have been chosen and are ready to hear testimony in the trial that is expected to last 15 weeks, according to the prosecutor's office.
Ortega faces two counts of first-degree murder and two counts of second-degree murder. If convicted, she faces life in prison.
Mother discovers a nightmarish scene
On a typical fall day in late October 2012, Marina Krim, the mother of Lucia and Leo, left her two children in Ortega's care while she took her third child, Nessie, then 3, to a swimming lesson at a nearby YMCA, police said.
Krim had made plans to meet Ortega, whom her children affectionately called Josie, at a dance class for Lucia at 5:30 p.m. When Ortega didn't arrive on time, Krim grew concerned and returned to her home to check on the children.
When she arrived, she discovered the lights were off, which puzzled her. She returned to the lobby and asked the doorman if he had seen her children leave. He hadn't.
"There comes a time when she goes looking for her children and enters the bathroom and finds her 6-year-old daughter and son stabbed to death in the tub," then-New York police Commissioner Ray Kelly said.
Krim and her daughter Nessie saw Ortega sitting on the floor next to the tub while she stabbed herself in what authorities believe was an attempt to take her own life.
The children's father and Marina's husband, Kevin Krim, was returning home from a West Coast business trip when police reached him at John F. Kennedy International Airport to share the tragic news.
Nanny was considered part of the family
Marina Krim kept an online blog in which she shared photos of her children and their experiences as a family. In it, she treated Ortega as a member of the family.
A native of the Dominican Republic, Ortega is a naturalized US citizen. At the time of the children's deaths, she was living in Manhattan with her son, sister and niece, police said.
Ortega was introduced to the Krims through a family friend and had been working with them for two years, police said.
By all accounts, the Krims were happy with Ortega's work.
In February 2012, nine months before the killings, the Krims traveled with Ortega to the Dominican Republic, where they spent nine days, and the family stayed at the home of Ortega's sister for part of the trip. In her blog, Marina Krim described the experience as wonderful.
Grieving family goes forward
Years after the deaths, Krim wrote a post about her experience.
"The weeks that followed were surreal. There were terrifying flashbacks, police inquiries and psychiatrist appointments. There was media attention, an apartment we could never return to, and a memorial service. All of this coupled with the overwhelming grief that always ended in the questions 'How did this happen? Why did this happen?' " she wrote.
The how and why still puzzle the Krims, but they haven't let this tragedy hold them back.
"Since Lulu and Leo died, we've had two more children, Felix and Linus. ... They, along with strong Nessie, are genetically and spiritually half Lulu and half Leo," Kevin Krim wrote in a blog post.
In memory of their slain children, the Krims created the Lulu & Leo Fund in what Kevin Krim called "an act of positive defiance." The fund is a curriculum-based call to action to help children and families foster creative confidence and build resilience.