Complex sentence: Hundreds of people were told by 911 operators to remain inside the towers, despite evacuation orders issued by fire commanders on the ground; the 911 operators, who work for the Police Department, did not know about the evacuation order issued by the Fire Department.
Split into simpler sentences: Hundreds of people were told by 911 operators to remain inside the towers, despite evacuation orders issued by fire commanders on the ground. The 911 operators, who work for the Police Department, did not know about the evacuation order issued by the Fire Department.