PERQUIMANS COUNTY, N.C. (WVEC) -- The National Transportation Safety Board (NTSB) released its initial findings of the investigation into a medical helicopter crash that killed four people.

The Duke Life Flight helicopter went down on September 8 on a wind turbine farm in Hertford.

Mary Bartlett, the patient who was on board, died along with pilot Jeff Burke and registered nurses Crystal Sollinger and Kris Harrison.

The NTSB's preliminary report said that the helicopter stopped transmitting data about nine minutes after takeoff.

The report states:

Several witnesses reported observing smoke trailing behind the helicopter while it was in flight. The smoke was described by some witnesses as "heavy" or "dark", while others reported the color as "black", "dark blue" or "blue." One witness reported that the helicopter was "hovering" and "not travelling forward" while it was a "couple of hundred feet" above the wind turbine farm. Another witness reported hearing a "popping noise," he then observed the helicopter turn left, then right. It then descended quickly and appeared "in control" with the rotors turning before he lost sight of it.

Investigators at the crash site found that the cabin collapsed downward and was consumed partially by a post-crash fire. Althought heat damaged the helicopter's on-board audio and video recording system, its memory device was intact. People at the NTSB vehicle recorder laboratory will examine the unit.

The most recent 30-hour inspection of the helicopter was done on August 15, with additional inspections taking place on September 1 during scheduled maintenance.

NTSB Duke Life Flight Crash Preliminary Report by 13News Now on Scribd