The comedian, who was 81, died Sept. 4 after she'd been hospitalized for about a week when she went into cardiac arrest during the procedure at a doctor's office.
Rivers died from "anoxic encephalopathy due to hypoxic arrest" – brain damage due to lack of oxygen – during a procedure to scope her upper gastrointestinal tract and vocal folds, a spokeswoman for the city's medical examiner said.
The death was classified as a complication of a medical procedure. The classification is not commonly used; more deaths are certified as homicides, suicides or natural causes.
Rivers had been sedated during the procedure with propofol, the medical examiner said.
The state Health Department also is investigating.
Rivers' daughter and TV partner, Melissa Rivers , said they had no comment on the ruling.
"We continue to be saddened by our tragic loss and grateful for the enormous outpouring of love and support from around the world," she said in a statement.
Rivers was a raucous, acid-tongued comedian who crashed the male-dominated realm of late-night talk shows. With her raspy voice and brash New York accent, she turned the red carpet of the Oscars, Emmys and Golden Globes into a stalking ground for E! Entertainment, where she first began working in 1994. Her familiar query – "Who are you wearing?" – would quickly give way to such snarky commentary as her assessment of Adele's Grammy outfit: The singer looked like she was sitting on a teapot.
In 2007, Rivers and her daughter were dropped by their new employer, the TV Guide Channel, and replaced by actress Lisa Rinna. But the Rivers women returned to E! and found new success with Fashion Police, which Rivers hosted and her daughter produced.