Ms Des Forges wrote a key study of the Rwanda genocide
The court trying alleged perpetrators of the Rwandan genocide has expressed its shock at the death in an air crash of a top expert on the 1994 massacres.
Alison Des Forges, 66, was among 50 people killed in a plane crash on Thursday near Buffalo, New York state.
A spokesman for the the UN tribunal for Rwanda called her death "a great loss", said AFP news agency.
Ms Des Forges was an expert adviser to the court on the genocide, in which some 800,000 people were killed.
"It is with deep shock that the tribunal has learned of the tragic disappearance of Alison Des Forges," said Roland Amoussouga, a spokesman for the International Criminal Tribunal for Rwanda.
"It is a great loss for the world of human rights, international justice and the whole of humanity," he added.
Ms Des Forges had testified as an expert witness at 11 genocide trials at the tribunal, which is based in Arusha, Tanzania.
She also wrote a key study of the Rwanda genocide, Leave None to Tell the Story, and was a senior adviser to US advocacy group Human Rights Watch.
Ms Des Forges was returning to her Buffalo home when the plane crashed
All 49 passengers and crew on board a Continental airways twin-prop plane and one person on the ground were killed late on Thursday when it crashed into a house just minutes before it was due to land at Buffalo's airport.
Investigators have said that the crew of the plane noticed "significant ice build-up" on its wings before it crashed.
The crew also discussed the snowy and misty conditions as they descended to land.
Analysis of data recorders recovered from the scene shows the plane made a series of severe pitches and rolls seconds before plunging into a house, the investigators said.
The plane was flying from Newark, New Jersey, to Buffalo. Ms Des Forges was returning to her home in Buffalo after briefing European diplomats on the situation in Rwanda and Africa's Great Lakes region.