Mexicantown Massacre is a major event prior to Deus Ex: Human Revolution.
Sometime prior to 2027, a mechanically augmented fifteen-year-old criminal posed a potential threat in Mexicantown. Adam Jensen was given the order to shoot to kill, but refused to carry it out, The order was passed on to his team commander, Wayne Haas, who followed it. This resulted in violent, city-wide riots.
As a result of this, Jensen chose to leave SWAT. His superiors, however, used him as a scapegoat, blaming him for the entire incident.
In Deus Ex: Icarus Effect, Anna Kelso is lectured by the Juggernaut Collective of possible events orchestrated by the Illuminati. The shooting of the augmented teenager in Detroit is mentioned, making it possible that the entire incident was part of the conspiracies' design.