A mother returned to her luxury New York apartment to find her two children stabbed to death and their nanny lying nearby with self-inflicted wounds, police said.
New York Police Department spokesman Paul Browne said the children, two-year-old Leo and six-year-old Lulu, suffered "multiple stab wounds," and were pronounced dead at the scene.
"It's about the worst thing you can hear or imagine," he added.
The 50-year-old nanny, named locally as Yoselyn Ortega, was found lying on the bathroom floor and had stabbed herself in the neck, according to police.
A bloody kitchen knife was found on the floor.
She has been placed under arrest and is in a critical but stable condition in hospital.
The children's mother, Marina Krim, had returned home with a third child, three-year-old Nessie, after the girl's swimming lesson, Police Commissioner Ray Kelly said.
She saw that the apartment was dark and returned to the lobby to ask the doorman if the nanny and children had gone out. When the doorman said no, she returned to the apartment and made the discovery.
Music therapist Rima Starr, who lives on the same floor as the family, said she heard screams coming from the apartment in a building just a block from Central Park on the city's affluent Upper West Side.
"There was some kind of screaming about, 'You slit her throat!'" she said. "It was horrible."
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.
Comcast and NBC Universal officials said the sadness felt for the Krim family is "without measure", according to a statement issued Friday.
"A member of the CNBC family has suffered an unimaginable loss," the statement read.
"The sadness that we all feel for Kevin, Marina and their family is without measure. Our thoughts, prayers and unwavering support are with them all."