She lay on the floor at the Brooklyn hospital for an hour before a nurse finally checked her pulse.
After an autopsy and weeks of tests, the medical examiner's office concluded Friday that Green was killed by pulmonary thromboemboli, blood clots that form in the legs and travel through the bloodstream to the lungs.
The medical examiner said the clots were due to "deep venous thrombosis of lower extremities due to physical inactivity," complicating an underlying psychological illness: chronic paranoid schizophrenia.
Green died while awaiting care in the hospital's psychiatric emergency room. EMS workers had brought her to the center on the morning of June 18. The hospital said she was suffering from agitation and psychosis and was involuntarily admitted after refusing medical review.
The emergency room is chronically overcrowded, and Green waited overnight for further care.
A recording of her death prompted national outrage when it became public last week.
After she collapsed, neither fellow patients nor the hospital's staff moved to help her, even as she thrashed her legs on the floor and tried to get up.
Two security guards and a member of the hospital's medical staff can be seen on the video, stopping to look at Green briefly before walking away. She stopped moving about 30 minutes after falling and was dead when a nurse finally examined her another 30 minutes after that.
An attorney for Green's family, Sanford Rubenstein, said the finding suggested that the hours she sat in the hospital factored in her death.
"The length of time that she spent in the emergency room ... very well may have contributed to her death," he said. "Physical inactivity was obviously a significant contributing factor."
He said that had Green been carefully attended to when she arrived at the emergency room, doctors might have noticed swelling in her legs and taken action.
People known to be at risk from deep vein thrombosis are often given anticoagulation drugs or compression stockings, which can keep clots from forming, and advised not to sit for hours at a time.
The condition, however, is not always easy to detect. The National Heart Lung and Blood Institute said about half of the people with deep vein thrombosis have no symptoms at all.
Airlines often advise passengers on very long flights to stroll the aisle, periodically, to prevent blood clots.
The city Health and Hospitals Corp., which owns Kings County Hospital, had no immediate comment Friday.
HHC officials have previously expressed outrage at the way Green was treated. Six employees lost their jobs over the incident, even before it became public.
The agency also immediately reported the death to the state and voluntarily turned over the security records to lawyers already suing the city over alleged patient neglect at the hospital.
