After months of demonstrations, on the eve of the 1968 Summer Olympics in Mexico City — a mere 10 days before the opening ceremonies — troops and police fired upon a group of unarmed student demonstrators in the center square in the city's Tlatelolco neighborhood. The shots prompted thousands of protesters to flee in a panic as tanks — some stationed at the Zócalo square, the heart of the Mexican capital — bulldozed their way into the plaza. Though original estimates reported very few deaths, less than 20 people, eyewitnesses described seeing hundreds of people killed and thousands more captured, beaten and jailed. Countless more disappeared. More than 40 years after the Oct. 2 massacre, the events of the day are still shrouded in mystery.