The personal doctor of Joan Rivers is being investigated for the death of the popular American comedian and television host who died a week after undergoing a minor throat operation.

According to a source cited by the CNN television network, the doctor performed a biopsy on Rivers' vocal cord for which he had no written authorisation from the patient or permission from the New York clinic where the surgery was performed.

The doctor also took a "selfie" while the 81-year-old Rivers was under anesthesia in the operation theatre.

Rivers had been admitted Aug 28 to the clinic where Lawrence Cohen performed the endoscopy, a procedure which was intended to discover the reason for her throat problems.

After the endoscopy, the physician did a biopsy without the knowledge of Yorkville Endoscopy clinic, which refuses to perform biopsies at its facilities.

Cohen was suspended from the clinic after the incident.

Investigators believe that Rivers' vocal cords became inflammed during the biopsy and that led to respiratory failure which in turn triggered cardiac arrest that killed the comedian.

Rivers was taken from the clinic to Monte Sinai hospital in New York City where she died Sep 4.

Joan Alexandra Molinsky, widely known as Joan Rivers, was born in Brooklyn, but grew up in the New York City suburb of Larchmont. She started writing for "The Tonight Show" in 1965, and got her own show in 1993.