In a massive coordinated terrorist attack 19 men hijacked 4 jetliners on the morning of September 11. The hijackers were on a suicide mission and had received orders to take over control of the aircraft and to fly the planes into specified targets. United Airlines Flight 175 departed Boston-Logan for Los Angeles at 08:14. The aircraft was hijacked by five terrorists. The hijackers took over control, reportedly switched off the transponder and changed course to New York. At 09:03 the aircraft was flown into the Southeast side of the 110-story New York WTC South tower in a sharp left wing low attitude, crashed and exploded into the 78th through 84th floors. According to an FAA study, N612UA struck the WTC at a speed of 586 mph; an MIT study however determined the plane was probably traveling at 537 mph. The massive fire weakened the tower structure on these floors and the tower collapsed at 09:59. Most of the office workers below the 78th floor were able to evacuate the tower on time before the collapse. Seventeen minutes before Flight 175 crashed into the South Tower, the WTC North Tower had been hit by an American Airlines Boeing 767, N334AA. A total of 2,606 people were killed in the World Trade Center buildings, including about 292 on the ground. A precise breakdown of the number casualties relating to Flight 11 (North Tower) and Flight 175 (South Tower) is impossible. It has been estimated that about 1600 people were killed on the North Tower, and about 900 on the South Tower.