By Anahi Rama and Lizbeth DiazMEXICO CITY (Reuters) - Mexico's army is withholding key evidence from international investigators in the case of 43 trainee teachers abducted and apparently massacred in late 2014, hampering their efforts to reach the truth, the experts told Reuters. More than 18 months after the incident and just one week before the team of experts' window to investigate closes, the army has still not handed over an undisclosed number of photographs and video taken by a military intelligence officer as police clashed with the students on Sept. 26, 2014, the five investigators said.When Reuters requested the original photographs of police rounding up a group of the students from the military, via a freedom of information request, the army responded that the evidence was "inexistent."The investigators have also not been allowed to question the soldiers on duty that night at Battalion 27, based in Iguala in the restive southwestern state of Guerrero."Access to Battalion 27 and its members is fundamental to the investigation ... The state needs to explain," said James Cavallaro, president of the Inter-American Commission on Human Rights (IACHR), which commissioned the panel of experts.Testimony given by soldiers to the attorney general's office shows the army was aware of the clashes and did not intervene."We have repeatedly asked the attorney general's office to get (the military) to give us photographs, videos and documents which have been referred to in their own testimony," said Francisco Cox, a Chilean member of the independent panel of five experts commissioned by the IACHR. "We made a list of people who needed to testify. Some haven't and there are others who ought to testify again," he added. "They have made declarations and the fundamental issues still haven't been answered."The panel members say the testimony the soldiers have given so far to the attorney general's office is flawed and incomplete, because the questioning was too basic and they see some discrepancies.Mexico's defense ministry, which is in charge of the army, did not respond to repeated requests for comment.Previously, the army has said there is no reason its soldiers should be interviewed by the team of international experts. "I can't permit them to treat soldiers as criminals or interrogate them and make it seem as though they had something to do with it," Salvador Cienfuegos, who is Mexico's defense minister and the head of the army, said in October.The attorney general's office declined to comment, but a source told Reuters the investigation would remain open and they have not called soldiers to give evidence again because they are preoccupied dealing with other parts of the case.IMPUNITYThe case has drawn fresh attention to police abuses and impunity in Mexico. Drug cartels often have local security forces in their pockets, government purges of police ranks have shown.The trainee teachers studied at a rural college in the restive state of Guerrero. About 100 were attacked in the town of Iguala on Sept 26, 2014 after they hijacked five buses to transport them to a march commemorating a massacre back in the 1960s.Forty-three disappeared and are believed to have been murdered. But just one of them has been identified from a charred bone and the team of international experts rejected a government assertion that the 43 were burned in a pyre at a garbage dump. The Mexican government said in January 2015 it believed that corrupt police working with a local drug gang murdered the students, citing testimony given by detained suspects. But relatives of the disappeared rejected that account, and accused the government of trying to close the case early.No-one has yet stood trial over the case.The government has said the gang mistook the youths for rival gang members, and that police handed them over to Guerreros Unidos henchmen, who then burned them to ashes at a garbage dump in the town of Cocula, in the hills near Iguala.President Enrique Pena Nieto agreed with the IACHR in late 2014 to allow the international experts to investigate the case and promised them access to all the information they needed.The experts, who examined the site, say the government's garbage dump fire account is scientifically impossible given the heat needed to reduce human remains to ash and charred bone fragments. The fate of the students, who were long seen by local authorities as troublemakers, is still unclear. Families and lawyers of the disappeared students believe the army was involved in their abduction, though no evidence has been presented to support this. "The army pitched in to form a cordon so the aggressors could act," said Vidulfo Rosales, one of the lawyers representing families of the victims. Since 2007, the army has been tasked with ensuring public security in some of the most violent corners of Mexico. However, lawmakers say there is no specific law that requires the army to intervene in such cases of unrest. One military intelligence operative, Eduardo Mota, testified to the attorney general's office in December 2014 that he snapped an undisclosed number of photographs as police threw tear gas at a group of the students in front of the local courthouse in Iguala and detained them. That group of students, which investigators number at about 17, was never seen again. The army has not handed over the original photographs he testified to taking to the experts, who say they believe Mota also filmed video, citing standard army operating procedures. Mota could not be reached for comment. Instead, officials later gave them a presentation that included a power-point containing four photographs from the night in question, one of the investigators said, but not the original files.(Editing by Simon Gardner and Kieran Murray)Join the conversation about this story Click here to read full news..