2002 Überlingen mid-air collision

The official investigation by the German Federal Bureau of Aircraft Accident Investigation (; BFU) identified the main cause of the collision to be a number of shortcomings on the part of the Swiss air traffic control (ATC) service in charge of the sector involved, as well as ambiguities in the procedures regarding the use of the traffic collision avoidance system (TCAS) on board.
On 24 February 2004, Peter Nielsen, the air traffic controller on duty at the time of the collision, was murdered in an apparent act of revenge by Vitaly Kaloyev, a Russian architect whose wife and two children died in the accident. Provided by Wikipedia