“There were bloodcurdling screams from a woman,” said Rima Starr, who lives down the hall from the victims’ second-floor apartment. Ms. Starr also recognized a man’s screaming voice as that of the building superintendent.The screams prompted neighbors to call 911. Ms. Ortega was arrested as soon as the police arrived. She was taken to NewYork-Presbyterian Hospital/Weill Cornell Medical Center, where she was in critical but stable condition.

Image Lucia Krim

According to the police, Ms. Krim and her husband, Kevin, had three children — Nessie, the 3-year-old who lived, and Lucia and Leo. Ms. Krim wrote a blog where she documented “life with the little Krim kids” and showed them in photos around New York City, eating Gray’s Papaya hot dogs, pretending to use a pay phone, napping on the sofa and picking pumpkins.

On the Upper West Side, with its dual-income families in which nannies are often an integral part of children’s lives, pushing strollers or walking their charges by the dozens home from school in the afternoon, the news of the double killing was met with stark disbelief.

“It’s family-oriented, this neighborhood,” said Pauline Sklar, a real estate investor who lives a block from the building where the children were stabbed. “Parents are working. They have to depend on people. My niece hires people. She researches them.”

Ms. Sklar paused, then added, “Or tries to.”

Nannies are there for meal times, for bedtime, for birthdays and holidays, and go on vacations. Indeed, on her blog, Ms. Krim described how she and her family had spent several days visiting Ms. Ortega’s family in the Dominican Republic, speaking to just how close her relationship had been with the family.