A powerful reading and viewing experience from John Branch and the New York Times.

The reporting for this article on the Feb. 19 avalanche at Tunnel Creek was done over six months. It involved interviews with every survivor, the families of the deceased, first responders at Tunnel Creek, officials at Stevens Pass and snow-science experts. It also included the examination of reports by the police, the medical examiner and the Stevens Pass Ski Patrol, as well as 40 calls to 911 made in the aftermath of the avalanche. The Swiss Federal Institute for Snow and Avalanche Research provided a computer-generated simulation of the avalanche, based on data accumulated from the Stevens Pass accident report and witness accounts. Additional sources are: LIDAR data from King County GIS Center; Iowa Environmental Mesonet, Iowa State University; Mark Moore, U.S. Forest Service; National Avalanche Center.