A New York nanny was charged with murder on Saturday, nine days after a brother and sister were found fatally stabbed in a bathtub in their family's Manhattan apartment, police said.

Yoselyn Ortega, 50, was arrested and charged with first- and second-degree murder in each of the October 25 stabbing deaths, New York police spokesman Paul Browne said.

Ortega had been recovering since then at Weill Cornell Medical Center from stab wounds authorities believe were self-inflicted. From the outset, police suggested they thought Ortega had killed Lucia Krim, 6, and her brother Leo, 1, and she has been under police guard at the hospital.

On Saturday afternoon, Ortega was interviewed at her bedside by New York police detectives about what happened last month in the Crim family's Upper West Side apartment. Afterward, the New York resident was formally arrested and charged at 6:06 p.m., though she remains hospitalized under guard, according to Browne.

After initially finding her apartment dark, the slain children's mother entered a bathroom and found Lucia and Leo dead. Ortega then began stabbing herself with a kitchen knife, police Commissioner Ray Kelly said last month. The woman's wrists were slit.

Marina Krim, the mother, had left the children with the nanny to take her 3-year-old child Nessie to a swim lesson at a nearby YMCA, Kelly said. She had expected to meet the nanny around 5:30 p.m. at a dance class, but when Ortega failed to appear she went to the apartment.

The late children's father is Kevin Krim, a senior vice president for CNBC Digital and former Yahoo executive.