A plane that crashed onto a house in New York state, killing 50 people, was on autopilot when it went down in icy weather, investigators have said.
The Continental Airlines pilot may have breached federal safety recommendations by flying in such conditions, investigator Steve Chealander said.
Pilots are recommended to fly manually in icy weather, he said.
Analysis of the plane's data recorders shows the crew noticed significant ice build-up on its wings before the crash.
"You may be able in a manual mode to sense something sooner than the autopilot can sense it," said Mr Chealander, a spokesman for the National Transportation Safety Board (NTSB).
He told the Associated Press news agency that the NTSB recommended pilots disengage their craft's autopilot facility in icy conditions.
Pitches and rolls
The Bombardier Dash 8 was approaching Buffalo's airport when it crashed flat on the house in Clarence Center, a suburb of Buffalo, last Thursday.
The pilots did not tell air traffic controllers of any problems during the flight, and a recording of communications appears normal until shortly before the crash.
At that point, the crew are heard discussing snowy and misty conditions and asking for permission to descend.
The crew then discussed "significant ice build-up" on the windshield and leading edge of the aircraft's wings, Mr Chealander said on Friday.
The plane's de-icing system had been switched on before the crew talked about the ice, he said.
Preliminary investigations suggested the plane was pointing away from the airport at which it was meant to land when it crashed.
Analysis of the data recorders shows the plane made a series of severe pitches and rolls seconds before plunging into the house.
The dead include 44 passengers, four crew and one off-duty pilot on board, and one person on the ground.
The twin-prop Bombardier Dash 8 Q400 aircraft, operated by Colgan Air for Continental Airlines, was flying from Newark airport in New Jersey to Buffalo Niagara International Airport.