Water-bottles were possibly thrown at the patrol car. The two officers left, they parked their car a couple of blocks away, they notified a squad of riot police that were in the area, and they proceeded to the square. There they start threatening and swearing at the group of kids - quite possibly (though this is still murky AFAIK) a different or larger group of kids - from a distance of possibly twenty meters. The kids swear back. It's like a street quarrel, only one side is armed and dangerous. No side moves towards the other. There are dozens of witnesses to all of this because the area is packed with cafes and shops. The police officer by all eyewitness accounts raises his gun, aims and shoots at a figure from the other side.