BALTIMORE (AP) — The ice protection system was not activated on a business jet that crashed into a Maryland house, killing six people, even though the air temperature was near freezing, according to a federal safety investigation released Wednesday.

The National Transportation Safety Board opened the public docket into the Dec. 8, 2014, crash in a neighborhood near a small airport near suburban Washington. The crash killed the pilot and his two passengers, along with a woman and her two young sons in the house.

NTSB spokesman Eric Weiss said the1,100-page report details all of aspects of the investigation into the crash. The next step is an analysis of the facts, then the finding of probable cause.

The crash sparked a fireball that gutted the family's home in Gaithersburg, Maryland. Police said Marie Gemmell died while trying to protect her sons, 3-year-old Cole and 1-month-old Devin, from the smoke and flames.

All three people on board the plane were from North Carolina and worked in the health care field, including the pilot, 66-year-old Dr. Michael Rosenberg.

The others aboard the plane were David Hartman, 52, and Chijioke Ogbuka, 31.

The plane, an Embraer EMB-500 Phenom 100, was equipped with an engine anti-ice system and a wing and stabilizer de-ice system, according to the docket.

A review of the airplane's flight data recorder found that the ice protection system was not activated at the time of the crash. The final outside air temperature recorded by the recorder was 1.75, degrees Celsius, or about 35.15 degree Fahrenheit.

Investigators found ice accumulation on the aircraft's nose, wing leading edges and horizontal stabilizer.

The airplane made an audible stall warning shortly before the crash, the report said. That warning would have come about 20 seconds earlier had the ice protection been activated, according to the report.