A mother returned home to her luxury New York City apartment building to find two of her small children stabbed to death in a bath and their nanny, with self-inflicted stab wounds, lying near them.
The nanny was taken to hospital in a critical condition and was in police custody. Authorities said she is suspected of killing the children.
The children's father, CNBC digital media executive Kevin Krim, who had been away on a business trip, was met by police at the airport on his return and was given an escort to the hospital.
Music therapist Rima Starr, who lives on the same floor as the family, said she heard screams from their apartment at about 5.30pm.
"There was some kind of screaming about, 'You slit her throat!"' she said. "It was horrible."
The children's mother, Marina Krim, entered the apartment with her three-year-old and at first thought her other two children were out with the 50-year-old nanny, police commissioner Raymond Kelly said.
She went downstairs and asked the doorman at her building on Manhattan's Upper West Side whether he had seen them leave.
When he said no, she went back upstairs and discovered her son Leo (1) and six-year-old daughter Lucia, known as Lulu, in the bathroom, Mr Kelly said. It was unclear how many times the children were stabbed.
The nanny was found on the bathroom floor with stab wounds to her neck, a kitchen knife close by, police said.
Mr Kelly said it was unclear how long the nanny had worked for the family.
Ms Starr said she believed the nanny had been hired just recently.
"I met her in the elevator, the day before yesterday, and was making small talk," she said.
After police arrived, she said, the mother remained in the building's lobby, screaming hysterically and clutching her surviving child.
The family had moved to New York from San Francisco within the past few years. The children's father was named general manager of CNBC's digital media division in March, after working previously in digital media at Bloomberg.
Their mother had a cooking blog and taught art classes to young children.
The family lived in a late 19th-century building where a three-bedroom unit will rent for $10,000 dollars (¤7,700) a month.
They had a greyhound, retired from racing, named Babar.