From Wikipedia

The Third Tower

The WTC complex comprised seven buildings, three of which completely collapsed on the day of the attacks. At 5:21:10 p.m., following the crumble of the east mechanical penthouse at 5:20:33 p.m., 7 World Trade Center, a 47-story steel-frame skyscraper across the street from the rest of the complex, became the third building to collapse completely. Unlike the Twin Towers, the collapse of 7 WTC had been anticipated for several hours and the building had been evacuated.