Having flown in the IST area for a few years, I can confirm everything that nowherespecial is writing.
Based on the official information so far, I would say this fatal accident is a very close copy of London a few years ago. Along the route the helicopter took there is a large area of heavy construction with lots of cranes and new buildings. I could see how the pilots were being pushed higher and higher to remain clear of these obstruction, to an altitude were they had very limited visibility horizontally, but were still able to see what's below them. The radio tower that they subsequently hit appeared out of nowhere. In this scenario, high speed leaves little margin.