CONTEXT:
The Tlatelolco massacre was a state crime and a crime against humanity, perpetrated on October 2, 1968 by the PRI Goverment Of Mexico (basically, the conservative goverment) during a rally organized by students in the Plaza de las Tres Culturas, located in the Nonoalco-Tlatelolco Housing Unit, in Mexico City.
The rally took place in the context of the student movement that had erupted on July 22, 1968, due to the repression of students by the police forces of the Federal District and military elements of the Mexican Army, following a fight between students from the Vocational Schools 2 and 5 of the IPN and the Isaac Ochoterena High School, incorporated into the UNAM. Those attending the rally were attacked by elements of the Mexican Army and members of the paramilitary group called the Olimpia Battalion, which, as it would be learned years later, was formed by members of the Presidential General Staff.
Days after the attack, the government and the media in Mexico claimed that government forces had been provoked by the protesters who fired at them, however, official documents published since 2000 suggest that the shooters had been employed by the government.
https://en.m.wikipedia.org/wiki/Tlatelolco_massacre