RALEIGH, N.C. — Witnesses saw dark smoke before a medical helicopter crashed in North Carolina, killing the four people on board, a preliminary report by the National Transportation Safety Board says.

One witness also said the Duke Life Flight helicopter was “hovering” and “not traveling forward” a couple of hundred feet in the air before it crashed Sept. 8 in a wind turbine field in the town of Hertford, about 160 miles (260 kilometers) east of Raleigh. Another witness heard a popping noise before the helicopter turned left, then right and descended quickly with the rotors turning before he lost sight of it.

Witnesses described the smoke as heavy or dark, or as being black or blue.

The on-board audio and video unit suffered heat damage while the memory device remained intact, the report says . It will be sent to an NTSB laboratory for examination.

The pilot advised at takeoff that the helicopter was departing with four people and about two hours of fuel on board. That was the only communication from the pilot, the report says.

Killed were nurses Kris Harrison and Crystal Sollinger; pilot Jeff Burke; and 70-year-old patient Mary Bartlett, whose family has said she needed treatment for complications from pancreatic cancer.

Bartlett’s husband, Lennard Bartlett Sr., told WAVY-TV a few days after the crash that he had wanted to fly with his wife. “I just said, ‘I’ll be there soon.’ I said, ‘I’m going to try to go with you if they’ll let me, but I’ll be there soon and I love you,’ and then I kissed her and she said, ‘I love you too.'”

The helicopter had departed Sentara Albemarle Regional Medical Center on its way to Duke University when it crashed 12 minutes after takeoff, the report says.

A memorial service for the four will be held Wednesday in Duke Chapel, followed by a tribute in the quad in front of the chapel.

Meanwhile, Duke University Hospital has grounded its remaining Life Flight helicopter until further notice. Hospital President Kevin Sowers said in statement that he has no other details beyond what’s in the NTSB report.

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Follow Martha Waggoner at http://twitter.com/mjwaggonernc