Mexico’s U.N. human rights commissioner told the parents of 43 students missing since 2014 on Wednesday that he admires their determination to fight for a fair trial amid a "climate of impunity."

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"We need to overcome this climate of impunity," Jan Jarab, the representative in Mexico of the Office of the High Commissioner for Human Rights, said in reference to the more than 27,000 people who have gone missing in Mexico over the past decade, many of whom, according to international rights groups, were victims of enforced disappearances.

“I want to make clear my admiration for your determination and what you have achieved," Jarab said. "Until now, the whereabouts of the 43 are unknown, and we have no guarantee that those criminally responsible will be punished in this case.”

During the closed-door meeting at a Guerrero school, Jarab hailed the fact that Mexico's attorney general's office was opening new lines of investigation. He also said a special monitoring mechanism dictated by the Inter-American Commission on Human Rights, or IACHR, would be installed soon to ensure the Mexican government is held accountable for its probe.

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His visit came days before the second anniversary of the tragedy, which took place in the nearby city of Iguala.

Dozens of students had traveled there to hijack buses for a protest in Mexico City, a common practice in their radical left-wing school, when they came under fire from local police.

The authorities said months later that the officers handed the 43 students over to a drug cartel, which killed them, incinerated their bodies at a garbage dump and tossed the remains in a river.

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But independent experts from the Inter-American Commission on Human Rights have issued two damning reports rejecting the government's conclusions, saying such a big fire was not scientifically possible . The experts also corroborated news reports indicating that federal police had been monitoring the students since they left Ayotzinapa for Iguala and, at the very least, knew they had come under armed attack yet did not intervene.

Last week, Eber Betanzos, deputy attorney general for human rights, told AFP that the authorities would conduct new searches for clues in other locations using laser scanning technology. They are also investigating the possible role of other local police departments in the disappearance.

"We have seen in recent weeks some indications that they are already investigating other leads," Jarab told reporters. "It seems like a positive thing to me, although we will obviously follow the process.”

Jarab gave his backing to legislation introduced in Congress aimed at combatting the large number of disappearances in Mexico.

"There are a lot of missing people and the Ayotzinapa case made it known," he said.