Source: Joan Rivers had surprise throat biopsy that led to her death

The 81-year-old suffered a heart attack and stopped breathing on August 28th during a routine endoscopy at a New York clinic. But the doctor who operated on her reportedly noticed something on her vocal cords and then performed the unplanned biopsy - something that isn't supposed to be done outside of a hospital.

The procedure is said to have caused Rivers' vocal chords to seize and cut off her air supply.

The Daily News source called it a "huge no-no" and said, "If she had been in a hospital when it happened, she might have been okay."

Rivers died a week after the incident and a state medical examiner completed an autopsy but was not able to determine the cause of death.

In 1985, Rivers appeared on Good Morning America, where she was asked about her heart arrhythmia, a condition in which the heart unexpectedly beats out of sync...something Rivers says scared the hell out of her.

"All of a sudden you'll be on stage and hear 'tha-thud!...It scares the hell out of me."

The State Health Department is investigating the clinic, but currently it is not a criminal investigation.

A source told New York Daily News an unplanned throat biopsy cut off her air supply and possibly killed Joan Rivers.