A woman holds a child’s hand next to a memorial outside the building where two children were stabbed to death by their nanny in New York. (AP)

Oct. 27: She was unravelling. Yoselyn Ortega’s home was an overcrowded tenement that she yearned to leave. She shared the apartment with her teenage son, a sister and a niece, and roamed the halls selling cheap cosmetics and jewellery for extra money.

She had been forced to relinquish a new apartment for her and her son and move back. A woman had chiselled her on a debt. Neighbours found her sulky and remote. She seemed to be losing weight.

Juan Pozo, 67, a car service driver who used to rent a room in her apartment, said he spoke to her sister yesterday, who told him that Ortega had not been feeling well lately, “that she felt like she was losing her mind”. He said the family had taken her to see a psychologist, an account shared by others, including the police.

This was the unfinished portrait that began to emerge yesterday of Ortega, the Manhattan nanny who, the authorities said, committed the unthinkable.

On Thursday evening around 5.30, police commissioner Raymond W. Kelly said, Marina Krim returned to her Upper West Side apartment with her 3-year-old daughter to discover her two other children, a 2-year-old boy and a 6-year-old girl, dead of knife wounds in the bathtub and Ortega slashing herself with the same bloodied kitchen knife used on the children.

Ortega, 50, survived, but the police have been unable to question her because she remains in the hospital in a medically induced coma, a deep stab wound in her throat. She has not yet been charged.

The authorities remain mystified over the motive. Paul J. Browne, the chief police spokesman, said family members had told detectives that Ortega “over the last couple of months was not herself”. “She was, according to others, seeking some professional help,” he said, adding, “There were financial concerns.”

Ortega, who the police said was a naturalised American citizen from the Dominican Republic, had worked for the Krims for about two years. She had been referred by another family, the police said, and did not come through an agency, which customarily does background checks. A law enforcement official said Ortega had had no previous brushes with the law, nor have detectives learned of any tensions in her relationship with the Krims.

“No fighting with the mom, the family, the kids,” the official said. “Everybody is looking for a reason here.” He added, “We’ve got nothing bad other than the fact that she killed two children.”

Yesterday, the sort of memorial with stuffed animals and flowers that has become sadly familiar in the aftermath of a city tragedy took shape outside the Krim apartment building, as parents pondered what to say to their own children.

Disbelief was pervasive in the neighbourhood.

“I don’t have words for something like that,” said William Davila, whose daughter is a fifth grader at Public School 87, which Lucia Krim, 6, had attended. The children’s father, Kevin Krim, was returning from a business trip when he was met by the police at the airport.

Krim learned that his youngest child, Leo, and his daughter Lucia, known as Lulu, had died and that the police had arrested the nanny with whom the Krims were so close that they had travelled to her home in the Dominican Republic. He is an executive at CNBC. Marina Krim did not work outside the home, but taught an occasional art class at the Museum of Natural History. On Thursday night, CNBC put the Krims up in a hotel.

Krim’s father, William Krim, 74, said the parents had not returned to their apartment. “I don’t know if they ever will,” he said. “I don’t know if I could.”

A spokeswoman for the Office of the Chief Medical Examiner, Dr Charles S. Hirsch, said Lucia had died of “multiple stab and incise wounds”, and Leo of “incise wounds of the neck”. They had been clothed, a law enforcement official said, suggesting that Ortega had not been bathing the children.

For about 30 years, according to neighbours, Ortega has lived in a six-storey tenement building at 610 Riverside Drive in the Hamilton Heights neighbourhood in Upper Manhattan. Before the nanny job, they said, she had worked in factories and as a cleaning lady. A neighbour said the sister she lived with was a taxi driver.