The massacre was preceded by months of political unrest in the Mexican capital, echoing student demonstrations and riots all over the world during 1968. The Mexican students wanted to exploit the attention focused on Mexico City for the 1968 Olympic Games. Mexican President Gustavo Díaz Ordaz, however, was determined to stop the demonstrations and, in September, he ordered the army to occupy the campus of the National Autonomous University of Mexico, the largest university in Latin America. Students were beaten and arrested indiscriminately. Rector Barros Sierra resigned in protest on September 23.
Student demonstrators were not deterred, however. The demonstrations grew in size, until, on October 2, after student strikes lasting nine weeks, 15,000 students from various universities marched through the streets of Mexico City, carrying red carnations to protest the army's occupation of the university campus. By nightfall, 5,000 students and workers, many of them with spouses and children, had congregated in the Plaza de las Tres Culturas in Tlatelolco.
The massacre began at sunset when army and police forces — equipped with armored cars and tanks — surrounded the square and began firing live rounds into the crowd, hitting not only the protestors, but also other people who were present for reasons unrelated to the demonstration. Demonstrators and passersby alike, including children, were caught in the fire; soon, mounds of bodies lay on the ground. The killing continued through the night, with soldiers carrying out mopping-up operations on a house-to-house basis in the apartment buildings adjacent to the square. Witnesses to the event claim that the bodies were later removed in garbage trucks.
The official government explanation of the incident was that armed provocateurs among the demonstrators, stationed in buildings overlooking the crowd, had begun the firefight. Suddenly finding themselves sniper targets, the security forces had simply returned fire in self-defense.
In October 1997, the Mexican congress established a committee to investigate the Tlatelolco massacre. The committee interviewed many political players involved in the massacre, including Luis Echeverría Álvarez, a former president of Mexico who was Díaz Ordaz's minister of the interior at the time of the massacre. Echeverría admitted that the students had been unarmed, and also suggested that the military action was planned in advance, as a means to destroy the student movement.
In October 2003, the role of the US government in the massacre came to light when the National Security Archive at George Washington University published a series of records from the CIA, the Pentagon, the State Department, the FBI and the White House which were released in response to Freedom of Information Act requests.
The documents detail: