A preliminary report by a federal agency says witnesses saw dark smoke before a medical helicopter crashed in North Carolina, killing the four people on board.

The report by the National Transportation Safety Board says one witness also said the Duke Life Flight helicopter was “hovering” and “not traveling forward” before it crashed Sept. 8 in Hertford in northeastern North Carolina, about 160 miles (260 kilometers) east of Raleigh.

The report also says the on-board audio and video unit suffered heat damage while the memory device remained intact.

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.

A memorial service for the four will be held Wednesday in Duke Chapel.