Nanny tragedy: Police search for a motive

(CBS News) Police have been unable to interview the nanny suspected of stabbing to death 6-year-old Lucia Krim, and her 2-year-old brother Leo, inside a New York City apartment building.

The nanny, Yoselyn Ortega, 50, is in police custody at the hospital, where she is in critical condition. Officials believe she slashed her throat and wrists.

New York City Police Commissioner Ray Kelly said so far, investigators have found no history of violence in Ortega's background.

"She has been employed by the family for approximately 2 years," he said. "She was referred to them by another family."

It was Thursday afternoon when the children's mother, Marina Krim, arrived at her apartment with her 3-year-old daughter, Nessie. Krim was looking for her two other children and the nanny.

Moments later, neighbor Rima Starr heard inconsolable cries.

"Just these unbelievable blood curdling screams," Starr said. "No words. Just primal, from the bottom of the gut, horrible screams."

Officials say it was Krim who found her children stabbed in the bathtub. Nearby was Ortega.

"The nanny began to stab herself as the woman entered the room -- entered the bathroom," NYPD Commissioner Kelly said. "She did it as the mother entered the bathroom."

Their father, Kevin Krim, an executive at CNBC, was notified as he returned from a business trip. Outside the apartment building, strangers paid their respects. One card read, "So unbearable, so unfair."

On "CBS This Morning: Saturday," senior correspondent John Miller spoke to Anthony Mason and Rebecca Jarvis about the investigation.

He said that the nanny's family told police she was under increased stress, was going through some emotional issues and seeking help.

The investigation continued with a search of Ortega's apartment to figure out where she was getting that help, with the hope that a doctor, or clinic worker could provide some answers as to what was going on in the background of her life.

Miller also theorized that, based on information available and on past cases, Ortega probably didn't just snap.

"She may have been planning this for weeks," he said. "But that still doesn't get to the why."

To watch Elaine Quijano's full report and John Miller's discussion about the case, click on the video above.