A team of scientists say intense rainfall likely played a major role in triggering the landslide that killed 43 people in the town of Oso.
The report released Tuesday said many other factors such as previous landslide activity also weakened the slope that collapsed on March 22.
The team said the slide, the deadliest in U.S. history, occurred in two major stages. A fast-moving mudflow remobilized a 2006 slide, bringing down old slide deposits across the valley. Another slide followed a few minutes later.
The report makes several broad recommendations that include doing an examination of landslide risks and communicating the information to the public.