Government officials at the time said armed dissidents provoked the deadly confrontation on Oct. 2, 1968 — 10 days before the start of the Olympics hosted by Mexico — by firing on police during a protest against Mexico's lack of democracy. Estimates on the number of people killed range from 38 to several hundred.
As Mexicans hold an annual march Thursday to mark the anniversary of the attack at Tlatelolco plaza, there is growing evidence backing up claims by student protesters that government operatives initiated the massacre.
The Supreme Court ordered the federal Attorney General's office to investigate how big a role the government played in the massacre, but the investigation by a special prosecutor has faltered because those in charge at the time of the attack refuse to testify.
Researchers say newly uncovered files with police names and the number of snipers may give new momentum to the case.
Investigators say there is evidence top officials, including then-Interior Secretary Luis Echeverria, knew much more than they originally claimed. Echeverria would become Mexico's president in 1970.
Some snipers moving from window to window above the plaza scurried through an apartment that belonged to Echeverria's sister-in-law, according to the documents.
Echeverria, who was in charge of Mexico's domestic security, has denied any direct involvement in the attack, and he and his assistants refused to comment to the AP. Called in for questioning last year by Special Prosecutor Ignacio Carrillo, Echeverria refused to talk, citing his constitutional right to remain silent.
"You think Echeverria didn't know that they were firing from the apartment of his sister-in-law? I don't think so," Maria de Los Angeles Magdaleno, an investigator for the special prosecutor researching the case, said in an interview with the AP.
Magdaleno leads a team of five government researchers combing through millions of uncatalogued documents. Among the team's recent discoveries is a list of the intelligence monitors for dozens of military units and police squads who showed up at the plaza, another indication the events were no secret to high-ranking officials.
Researchers have uncovered mounting evidence that among those at the plaza was Manuel Diaz Escobar, who led a secret battalion of police known as the Falcons. The Falcons went on to lead another bloody attack on student protesters in 1971.
Magdaleno says there were diverse groups of police at the plaza, including federal police, city police and secret service agents.
Investigators do not have evidence the government had previously ordered police to carry out the killings, but documents suggest things got out of control.
"The decision was not to go out killing students and certainly not with the eyes of the world on Mexico" before the Olympics, Magdaleno said. "There were so many groups in the operation that the authorities couldn't control them."
Until recently, prosecutors had brushed aside demands to prosecute those responsible for the massacre, claiming the statute of limitations had expired years ago.
But on the anniversary of the massacre two years ago, President Vicente Fox promised to open secret government archives about the event.
Carrillo, the special prosecutor appointed by Fox to investigate crimes by past governments, said there is evidence 38 people were killed at Tlatelolco Plaza. Human rights groups have contended several hundred died, but government officials hid the bodies so an exact count was impossible. Government officials also seized photos of the carnage and ordered newspapers not to print photos of corpses.
International observers worry the investigation will stall, especially as government officials remain silent.
"If you don't put a limit on time, the process will go on for 10 years," said Sofia Macher, a member of Peru's Truth and Reconciliation Commission, which investigated deaths and disappearances in Peru during a brutal insurgency by Shining Path guerrillas.
Human Rights Watch has urged Fox to make the investigation a higher priority and to provide investigators with more resources. The group also wants Congress to pass new legislation that would pressure witnesses to testify, saying prosecutors need the power to negotiate plea bargains.
"As this thing drags out, I think the frustration has grown considerably," said Daniel Wilkinson, an attorney for Human Rights Watch. "It will take showing concrete results for that to abate."
At an appearance Tuesday by Fox, a group of students told the president they hadn't forgotten about the case.
"I haven't forgotten about it either," local newspapers quoted him as responding.