MEXICO CITY, Oct 1, 2008 (AFP) - Mexico commemorates the 40th anniversary of a deadly clampdown on student protestors this week with the details of the massacre still unclear, the perpetrators untried, and impunity as widespread as ever.

As student movements shook the world in 1968, Mexican security forces killed at least 44 protesting students in the country's capital 10 days before the start of the Olympics here, and rapidly cleared up the evidence.

It was just one peak in the country's history of impunity, apparent today in escalating drug-related violence in which some 3,000 have died so far this year and corrupt officials are often involved.

'They have not managed, neither in modern times nor with the lamentable experience and open wound of the movement of '68, to finish or diminish to more logical levels the country's impunity,' Jose Antonio Ibanez, coordinator of the Human Rights program of the Ibero-American University told AFP.

At 6 pm on October 2, 1968, hundreds of Mexican soldiers sprayed bullets on a demonstration of some 8,000 students gathered in the Square of Three Cultures, or Tlatelolco, in Mexico City, following more than a month of protests.

The mayhem lasted until midnight, leaving an undetermined number of dead among students, neighbors, soldiers and police, and 2,360 detained.

The next morning, a presidency spokesman said 20 had died and 70 had been hurt, as international media and the CIA registered some 300 deaths. Then president Gustavo Diaz Ordaz later said there were between 30 and 40 deaths.

The next five presidents maintained silence on the issue until 1998, when a legislative commission pinned the blame on Luis Echeverria Alvarez -- a former president who was minister of the interior at the time of the massacre -- without further consequences.

In 2003, then president Vicente Fox set up a commission to investigate past crimes and Echeverria was later put under house arrest -- but for crimes during the country's so-called 1970s 'dirty war,' not for the massacre.

Diaz Ordaz died in 1970, and Echeverria has lost his sense of reality due to his advanced age, said Jacinto Rodriguez, author of '1968: All the Guilty,' a new book on the massacre.

'Most of the soldiers involved are now dead,' Rodriguez added.

As preparations grow for Thursday's anniversary, many are skeptical that current President Felipe Calderon, who is leading a massive army crackdown on drug-related violence, will help shed any more light on the issue.

'Calderon's government doesn't talk about it. I don't think it's ready to collaborate,' said human rights defender Sergio Aguayo, adding that some key documents on the massacre had disappeared.

Some are now calling for a national truth commission to finally let Mexicans deal with their past.

Mexicans should seek 'justice from the true story and settle society's wound,' Rodriguez said.