In October 1968, Mexico was preparing to host the Olympics. It was a big moment for the country, as this would make it the first Latin American country to do so. At the same time, however, student protests were rocking cities worldwide – and Mexico City was no exception. On October 2, 1968, just ten days before the Olympics were set to begin, army troops opened fire on a peaceful student demonstration.

The official announcement was that only four students were killed but eyewitnesses say they saw hundreds of dead bodies trucked away. It was one of the darkest moments in Mexican history – and for decades after, the death toll wasn’t the only thing the government covered up.

Click here for a photo slideshow of the Mexican student movement and the Tlatelolco Massacre of 1968.

Below, watch archival footage recorded by the Mexican government on the day of the Tlatelolco massacre. Released more than twenty years afterwards, this footage reveals a secret about how the massacre began. A special army unit, disguised as civilians and positioned in an apartment building above the protests, fired shots down at the troops on the street, causing the troops to attack the student protesters.

Please be advised that this footage contains graphic content.