On September 11 , 2001 , American Airlines Flight 11 and United Airlines Flight 175 were hijacked by Islamic terrorists affiliated with Al Qaeda and flown into the World Trade Center 's North and South Towers , respectively , causing massive damage to both towers during impact , and starting fires that caused the weakened 110-story skyscrapers to collapse within less than two hours
United Airlines Flight 175
United Airlines Flight 175
New York City Fire Department