NEW YORK (CBSNewYork/AP) — The mother of two children allegedly murdered by their nanny in their Upper West Side home took the stand Thursday the first day of her trial.

In was in October of 2012 that prosecutors say 55-year-old Yoselyn Ortega stabbed 6-year-old Lucia Krim and 2-year-old Leo Krim to death in the family’s apartment. 

Their mother, Marina Krim, returned home with the couple’s third child to find Lucia, nicknamed “Lulu,” and Leo dead in a bloody bathtub. Investigators say Ortega had also slashed herself trying to take her own life, claiming she heard voices telling her to kill.

Ortega’s attorneys are mounting an insanity defense.

nanny1 She Killed My Best Friends: Trial Begins For Upper West Side Nanny Accused Of Killing 2 Kids

Artist rendering of the scene at the murder trial of nanny Yoselyn Ortega (Photo by Jane Rosenberg)

Marina Krim began her testimony by asking to take a good look at Ortega.

“You are totally out of this world,” Krim said to Ortega, before turning to the court and saying “She’s a liar!”

Krim asked the court for patience as she began her testimony, CBS2’s Hazel Sanchez reported.

“I’m trying really hard to get through this. I need you to be patient with me,” Krim said.

She described in heartbreaking detail how she realized something was terribly wrong when she went to pick up LuLu from ballet class and she wasn’t there. She rushed to their Upper West Side home with her 3-year-old daughter, Nessie. Hand in hand, they made the gruesome discovery.

“It’s like a total horror movie. I walk down the hall and I see the light on under the door,” she said. “I see LuLu. I knew that she was dead. She’s lying in the bathtub and her eyes are open. I see Leo next to her. They had blood on them.

“Then, I see the defendant – blood all over her and eyes bugging out.

“All I remember saying to her is, ‘I hate you!’”

nanny She Killed My Best Friends: Trial Begins For Upper West Side Nanny Accused Of Killing 2 Kids

Yoselyn Ortega (credit: CBS2)

Lulu tried to defend herself, prosecutors said.

“She knew what she was doing, she knew what was happening, she understood what the defendant was doing, she fought to live,” the prosecutor said. “And the defendant repaid LuLu’s resistance with almost 30 different stab and slash wounds to her body and her neck.”

Some jurors were in tears during her testimony, Sanchez reported.

“She killed my best friends,” the devastated mother said.

When leaving the courtroom, Krim turned to Ortega and said, “You’re gross. You’re disgusting.”

Earlier, the prosecutor began opening statements by saying that Yoselyn Ortega “viciously and violently” attacked the two.

“[She] repeatedly slashed and stabbed them, laying their bodies on top of one another to bleed out in a bathtub,” the prosecutor said. “The defendant brutally butchered both children, slitting their throats.”

Watch: Prosecutor’s Opening Statement 

Marina Krim returned home to make the horrifying discovery, prosecutors said.

“Marina Krim opened the bathroom door and saw the bloody, lifeless bodies of her 6-year-old daughter and 2-year-old son, stacked in the bathtub, their eyes open, covered in blood, staring ahead. And between her and her dead babies was the defendant, the person responsible for this atrocity, standing, holding one of the knives that she had used to kill those babies,” the prosecutor said.

“She collapsed on the landing sobbing uncontrollably and inconsolably that her children were dead,” the prosecutor said.

Ortega had cut her own throat, prosecutors said.

Ortega sat emotionless as prosecutors detailed the unthinkable murders of the two beautiful children who were under her care, Sanchez reported.

Prosecutors argue Ortega, 55, who had been working with the family for two years, was angry at the Krims for being too demanding, and that she resented Marina Krim for being able to provide for her children in a way that she could not, Sanchez reported.

“These are the undisputed facts in this case. And these are the facts and the evidence will prove to you beyond a reasonable doubt that the defendant intentionally murdered Lulu and Leo Krim on Oct. 25, 2012. That she knew what she was doing when she killed them and that she knew what she was doing wrong,” the prosecutor said.

During the two years Ortega worked for the family, Krim said Ortega never complained about any struggles that would lead her to that horrible fall day.

Watch: Defense’s Opening Statement 

Defense attorney Valerie Van Leer-Greenberg told the jury during her opening statement that Ortega suffered from severe mental illness, including “major depression, psychotic thinking, hallucinations.”

Van Leer-Greenberg added her client had not been treated for her problems until after her arrest. She said that from 1962 until 1982, when she lived in the Dominican Republic, her native land treated mental illness as if it was incurable.

“The evidence will show that she has a corroborated history of hearing voices and disassociating from reality, since the age of 16,” she said.

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 mentally fit to stand trial.

Prior to the start of the trial, Kevin and Marina Krim had been anticipating what will likely be excruciating for them — coming face to face with their former nanny.

“Over the next few months, the story of Lulu and Leo and our family will be painfully in the news again,” Kevin Krim said in a video message the couple released. “This trial will be very hard for us and for a lot of you.”

The Krims, who have since had two more children, have created a non-profit. The “Lulu and Leo Fund” is an organization that fosters creativity among children in memory of the two little ones they lost, using their tragedy to help others.

“We’re gonna handle this the way we’ve handled everything,” said Marina Krim. “We’re gonna focus on the positive and the goodness that’s come out of all this.”

The trial is expected to last at least three months.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)

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