NEW YORK -- A nanny who argued she was too mentally ill to be held responsible for stabbing two children to death at their home has been convicted. Jurors on Wednesday found that Yoselyn Ortega killed 6-year-old Lucia Krim and 2-year-old Leo Krim in October 2012 and understood the consequences of her actions when she did it. They said she was guilty of murder.

Ortega's lawyer Valerie Van Leer-Greenberg didn't dispute that Ortega killed the children but contended she had an undiagnosed mental illness that got worse in the moments leading up to the attack.

Mental illness "does not announce itself like a bad cough or a limp," Van Leer-Greenberg said during closing arguments. "Sometimes it sneaks up and nestles in before anyone takes notice."

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But prosecutors maintained that Ortega knew what she was doing and acted out of jealous hatred of the children's mother, Marina Krim, who was richer and happier than she was.

Leo and Lucia Krim CBS New York

"She did it intentionally with a full understanding of exactly what it was she was doing - every stab, every slash," Assistant District Attorney Stuart Silberg said during closing arguments.

Ortega, who's from the Dominican Republic, showed little emotion during the trial and mostly stared straight ahead. But she shook her head forcefully and mouthed "no" during testimony that her employers treated her well.

Jurors heard heart-wrenching testimony from Marina Krim, who spoke of the sickening, desperate moments when she saw her children's vacant eyes, their small bodies perforated by stab wounds.

Krim was at a swimming class with her 3-year-old daughter, Nessie. They went to pick up Lucia from dance class, but she wasn't there.

Krim, after finding Lucia's and Leo's bodies in the family's home, ran outside with Nessie, called for help and started screaming.

"It was a scream you can't imagine is even inside of you," she testified. "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.'"

Lucia, who was called Lulu, was stabbed more than 30 times and had defensive wounds on her body. Leo was stabbed five times.

Their father, Kevin Krim, had been away on a business trip. At Ortega's trial, he spoke of walking down a long hallway at the hospital where he saw his children.

"They still had this perfect skin and these long eyelashes," Krim said. "They had like sandy brown hair. ... You could see they tried really hard to wash all the blood out, but there was still kind of an auburn tint to it that I remember to this day."

The father's testimony brought tears to the jurors and to people seated in the courtroom gallery.

Ortega, who's 55 years old, faces life in prison.