Mexico is supposed to be a modern country, a democracy, a society under the rule of law. That is certainly what it aspires to be, and when President Enrique Peña Nieto took office in 2012 promising to aggressively tackle the violence that has disfigured Mexico in recent years, the hope was that those aspirations would be more fully realised than in the past. How then can it also be a country where the disappearance and presumed murder of 43 college students, whose only crime appears to have been that they hijacked some buses to attend a protest meeting, is still a mystery more than 18 months after it occurred? And not only a mystery, but one that the Mexican government seems determined will remain unsolved.

True, the government put out a version of what happened to these young men in Iguala over a year ago. But it was internally inconsistent, lacked proof of its main assertions, satisfied hardly anybody and was rejected by the parents of the students. Under pressure, Mexico then agreed that a panel of experts appointed by the Inter-American Commission on Human Rights would re-examine the students’ disappearance in collaboration with the government.

The panel’s first report cast additional doubt on the government story. Its second and final report, this week, does not solve the mystery but makes it clear that the failure to do so can largely be laid at the door of the uncooperative and obstructive Mexican authorities. The panel was unable to properly test the hypothesis that the students were killed because they had inadvertently taken a bus stacked with heroin, and were attacked by police and soldiers trying to retrieve the drugs on behalf of traffickers.

The Mexican investigators, it says, tortured the principal witnesses, failed to follow up evidence that pointed in a different direction from that which they had already decided was the only permissible one, and did not explore the possible culpability of higher level – including federal – officials.

The experts repeated earlier complaints that they had not been given access to important witnesses, had not been allowed to re-interview witnesses, and had not been able to see documents they had requested. And the government seems to have done nothing to discredit what looked very much liked a smear campaign in the Mexican media against two of the panel’s members. Their mandate will soon expire and the government has said it will not renew it.

This is a country with a homicide rate of almost 100 a day, an epidemic of disappearances, kidnappings on the rise, and one in which the killing of journalists is a commonplace. Another was shot this week in Guerrero state, the same state where the students died, although there is no suggestion of any connection. He was the fifth to be murdered this year. The Iguala case is not the only one in recent times where many died or are presumed dead in a single incident – there was another in Tlatlaya, also in Guerrero, in the summer of 2014 – but it was the worst.

There could not have been a better opportunity for the new start the government had proclaimed. Instead it invited in the most neutral body of experts that could be mustered and has now in effect expelled them with their work only half done. A more dismal result for Mexico can hardly be imagined.