Almost lost in the chaos of the collapse of the World Trade Center is a mystery that under normal circumstances would probably have captured the attention of the city and the world. That mystery is the collapse of a nearby 47-story, two-million-square-foot building. Engineers and other experts ... were for weeks still stunned by what had happened to 7 World Trade Center. [They] struggle to explain the collapse. That building had housed, among other things, the mayor's emergency command bunker. It tumbled to its knees shortly after 5:20 on the ugly evening of Sept. 11. Experts said no building like it, a modern, steel-reinforced high-rise, had ever collapsed because of an uncontrolled fire, and engineers have been trying to figure out exactly what happened and whether they should be worried about other buildings like it around the country. Even though Building 7 didn't get much attention in the media immediately, within the structural engineering community, it's considered to be much more important to understand," said William F. Baker, a partner in charge of structural engineering at the architectural firm Skidmore, Owings & Merrill. "They say, 'We know what happened at 1 and 2, but why did 7 come down?'" Jonathan Barnett, a professor of fire protection engineering at the Worcester Polytechnic Institute, said a definitive answer to the question of what happened in 7 World Trade Center is perhaps the most important question facing investigators. "It's just like when you investigate a plane crash. If we find a weakness in the building or a deficiency in the building that causes that collapse, we then want to find that weakness in other buildings and fix it."