NEW YORK — The nightmarish case of a nanny accused of stabbing to death two children in her care stunned the family’s well-to-do neighborhood and caused legions of parents to wonder how well they know who is watching their kids.

The nanny, Yoselyn Ortega, lay in critical condition Friday with what police said were self-inflicted knife wounds, and investigators were unable to question her, in part because she was still breathing with the help of a tube.

Her motive and mental state remained a mystery, and no immediate charges were filed.

On Thursday evening, the children’s mother, Marina Krim, took her 3-year-old daughter home from a swim lesson to find her other youngsters, ages 2 and 6, dying of knife wounds in the bathtub of their Upper West Side apartment near Central Park. Ortega then turned the blade on herself, police said.

Police Commissioner Raymond Kelly said the investigation has yet to reveal anything amiss in the household before the slayings.

Police were looking into whether Ortega, a 50-year-old naturalized U.S. citizen who had worked for the family for two years, had recently sought psychiatric help. Detectives were searching her home in Washington Heights, a working-class neighborhood north of where she worked and near Harlem.

If there was tension between the nanny and the Krims, it didn’t show on a Web journal kept by the children’s mother. Marina Krim spoke lovingly in one entry about traveling to the Dominican Republic last February to stay for several days at the home of Ortega’s sister.

“We met Josie’s amazing familia!!! And the Dominican Republic is a wonderful country!!” she wrote.

Pictures posted on the blog showed the two families posing together for a happy photo, with Ortega hugging the 3-year-old, Nessie, their cheeks pressed together.

Marina Krim, whose husband, Kevin Krim, is a CNBC digital media executive, wrote that Ortega’s family had nicknamed little Nessie “Rapida y Furiosa,” (or Fast and Furious), for her exuberance and energy.

There are tens of thousands of nannies working in New York City, but reports of serious violence by caregivers against children are exceedingly rare. Parents are accused of killing their own children with far more frequency.

More common are stories about nannies like Brunilda Tirado, who threw her body over a stroller to protect a baby from falling debris during a building collapse in the same Manhattan neighborhood in 2005. She suffered a broken arm and other injuries.

The slayings will undoubtedly prompt many parents hiring a nanny to check references more thoroughly and swallow hard over the possibility that they might unknowingly hire a person who would do their child harm.

“For working parents this is a nightmare.” said Denise Albert, who has two young children and lives a few blocks from the site of the tragedy.

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