The Similars

The film makes obvious references to the 1968 Olympics and the Tlatelolco Plaza massacre. I just can't make the connection between the film's story and the real life horror in Mexico City in October 1968. I hope somebody on this board can help me figure this out.

The "Similars" all look like archetypal student revolutionaries, and a key character repeats the line, "Kill all the innocents." Most of the government's victims in Tlatelolco Plaza were clearly innocent of any crimes meriting their execution by the Mexican government. Maybe the "rain" refers to a "rain of bullets" or a "reign of terror"? Another character asks if the mystery is the work of "the Americans or the Soviets," and we know today that the U.S. government was involved in the actions of the Mexican army at Tlatelolco. I just need help putting this all together.