Carrie Fisher Died from Cardiac Arrest in the Hospital

Carrie Fisher suffered cardiac arrest Tuesday morning and that's what killed her ... sources close to the family tell TMZ.

We're told Carrie was on life support after suffering what we were told was a massive heart attack Friday on a United flight. A family source tells TMZ they now believe the in-flight medical emergency was not a heart attack ... it was cardiac arrest, meaning her heart just stopped beating.

We're told as soon as she arrived at UCLA Medical Center she was placed on a ventilator.

As we reported, Carrie never improved. We're told the family was talking about removing life support, but early Tuesday morning, her heart just stopped, and she died.

Passengers on the United flight told TMZ at the time they thought she had died on the plane. She didn't have a pulse for 10 minutes and when she was removed her body appeared lifeless.