Marco Antonio Santiago Castro died that day at 12:54 hours, at Posta Nº 3 in Santiago, due to craniocerebral trauma, posterior thoracic contusions, and traumatic pulmonary hemorrhages, as certified by the Medical Death Certificate from the Legal Medical Institute.

According to witness statements and other records from the judicial proceeding initiated regarding his death in the Second Criminal Court of Santiago, on the morning of May 21, 1984, Marco Santiago was detained on suspicion by officials of the Investigaciones Police in the vicinity of the flea market located at the intersection of Calle Cumming and Calle Balmaceda.

At the time of his arrest, he resisted, for which he was beaten by his captors. Due to the blows received, he had to be transported immediately to Posta Nº 3 in Santiago, a facility he entered already deceased.

The Autopsy Report and its addendum concluded that "the injuries found, taken together, are necessarily fatal and contemporaneous, more characteristic of the actions of third parties" and that, due to their characteristics, "they were caused, rather, by personnel trained in that type of violence."

In the judicial investigation, the three arresting officials were prosecuted as perpetrators of the crime of unnecessary violence resulting in the death of Marco Santiago. However, they were acquitted in the final judgment.

Considering the evidence gathered and the investigations carried out by this Corporation, the Superior Council reached the conviction that the death of Marco Antonio Santiago Castro was a direct and immediate consequence of the beatings to which he was subjected by State agents during his detention. For this reason, it declared him a victim of human rights violations.