 Complete the following phrase:  Six World Trade Center was first proposed in 1968 as part of the original World Trade Center complex. The building was designed by Emery Roth  Sons, along with Yamasaki Associates. Construction was completed in 1973. The original Six World Trade Center was home to the U.S. Customs Service for the state of New York, from 1974 to 2001. It was a 537,693-square-foot 49,953-square-meter, eight-story building, the shortest in the World Trade Center complex. From the Austin J. Tobin Plaza level, on which the main structure was built, it had a height of 92 feet 28 m but was 105 feet 32 m above the actual ground level. Six World Trade Center was connected to the North Tower via underground entrances at the concourse and plaza levels and a small pedestrian walkway that extended from the south promenade of Six World Trade Center to the North Tower on plaza level. The Vesey Street Bridge which was connected to the World Ficial Center was directly connected to Six World Trade Centers main building structure on plaza level, which connected both the World Ficial Center and the World Trade Center together. Another two pedestrian walkways which both were connected to Seven World Trade Center were connected to the north promenade of Six World Trade Center. It was demolished following the destruction of the World Trade Center during the September 11 attacks, when 10 al-Qaeda participants hijacked American Airlines Flight 11 and United Airlines Flight 175, crashing them into the Twin Towers. After burning for less than two hours, both 110-story towers 