The slayings apparently stemmed from a domestic dispute, but investigators released no details.
Found dead at the scene were the suspect's girlfriend, their 1-year-old daughter, and the woman's mother and brother, said Col. John Fortunato, a spokesman for the Jefferson Parish sheriff. Another boy, 10, was treated for burns.
Firefighters responded to the fire about 3:45 a.m., but three of the victims had injuries caused by both stabbing and fire, Fortunato said.
Suspect Cyrus Casby, of nearby Marrero, has confessed, Fortunato said. He was booked with four counts of first-degree murder, one count of attempted first-degree murder and aggravated arson.
Casby's girlfriend, 17-year-old Cynthia Carto, died at a local hospital with fire-related injuries and stab wounds to the face and back. A daughter she had by Casby, 1-year-old Cyanna Carto, was dead at the scene with fire injuries.
Also dead at the scene were Cynthia Carto's mother, Janice Carto, 33, and brother, Cleveland Carto, 11, both of whom had stab wounds and fire-related injuries, the sheriff's office said.
Jarvis Carto, 10, a son of Janice Carto, was treated for burns at a Jefferson Parish hospital.
Autopsies were underway, Fortunato said.
The burned two-story apartment was one in a series of similar units located in a subdivision surrounded by shopping centers in Harvey, across the Mississippi River from New Orleans. Firefighters said the blaze started on the second floor.
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