2006, seven men were arrested by the FBI and charged with plotting to destroy the tower. Deputy FBI Director John Pistole described their plot as "more aspirational than operational". The case went to court in October 2007. After three trials, five of the suspects were convicted and two acquitted. The alleged leader of the group, Narseal Batiste, was sentenced to years in prison. In response to the perceived threat of an attack, the building's largest tenant at this time, Ernst & Young, moved to North Wacker Drive in early 2009. In May 2020, heavy rains caused three of the basement levels to flood, knocking out power to the building. This also resulted in many TV and radio stations going off the air. Architecture The Willis Tower was designed by architect Bruce Graham and structural engineer Fazlur Rahman Khan of Skidmore, Owings and Merrill. Graham and Khan designed the building as nine square "tubes", clustered in a 3×3 matrix forming a square base with sides. The building's total area is . The structure was intended to accommodate 16,500 employees. Form and facade Each of the "tubes" is a column-free module measuring , which set back at different stories. There are setbacks at the 50th, 66th, and 90th floors. The lowest 50 stories contain nine tubes and cover each. The northwest and southeast tubes terminate at the 50th floor. The 51st through 66th floors each span , above which the northeast and southwest tubes end. From the 67th to 90th floors, each story is shaped like a cross, covering . The north, east, and south tubes end at the 90th floor; the remaining west and center tubes reach 108 floors, with an area of on each of the top stories. The Sears Tower was the first building to use this innovative design. It was both structurally efficient and economic: at 1,450 feet, it provided more space and rose higher than the Empire State Building and cost much less per unit area. The system would prove highly influential in skyscraper construction and has been used in most supertall buildings since, including the world's current tallest building, the Burj Khalifa. In February 1982, two television antennas were added to the structure, increasing its total height to . The western antenna was later extended, bringing the overall height to on June 5, 2000, to improve reception of local NBC station WMAQ-TV. The perimeter of the