The helicopter did not fall off the building. There are pictures showing the ship tipped on its side. The pilot and passengers inside the helicopter escaped serious injury, as the fuselage protected them from the flying helicopter blades. However, the passengers who were outside on the helipad itself, as well as on the helicopter stairs (I believe there was one person standing there at the time) were decapitated almost instantly. In addition, pieces of helicopter blade flew off the side of the roof and fell to the avenue below. One piece struck a local woman walking on the ground and killed her as well.
I imagine the accident was much like the helicopter crash on ER
a few weeks back, except much more gruesome.