Lagos - The trials of more than 1 600 suspected Boko Haram members have begun behind closed doors at a military barracks in northern Nigeria, the largest mass trial in the Islamic extremist group's history.Nigeria's government is trying to show it is making progress against the group that has killed more than 20 000 people during its eight-year insurgency, but lawyers and rights groups are expressing concern about whether the suspects will have fair trials.The proceedings began on Monday at the Wawa Military Cantonment. In Nigeria, such trials are often held behind closed doors to prevent sensitive security issues from being made public.But the government may have other security concerns, Sylvester Odion-Akhaine, an associate professor of political science at Lagos State University, told The Associated Press."The government seems to have chosen a military facility to prevent an attack by Boko Haram," he said. The extremists in the past have attacked military barracks and prisons in efforts to free detained fighters.Fairness questionedLagos-based human rights lawyer Princewill Akpakpa expressed concern over whether the suspects will have proper trials behind closed doors. "If human rights organisations, lawyers and observers cannot access the trial venue, the fairness of the process will be put into question," Akpakpa said.Justice Binta Nyako said the trials will be fair. "We are here to ensure that nobody is persecuted. We are here for prosecution and so we have come with open minds," she said.Boko Haram has yet to comment publicly on the mass trials.Nigeria has arrested thousands of suspected Boko Haram members in recent years, and military detention facilities are overcrowded. Human rights groups say most of those detained have been picked up at random and without reasonable suspicion, including women and children.The trials will help decongest facilities that were not designed to hold so many suspects, Akpakpa said.Humanitarian crisis "Our cells were overcrowded with poor sanitary conditions and we were poorly fed," a former detainee who was held for four years at a military barracks in Maiduguri city told the AP. Some people died while in detention, he said. He spoke on condition of anonymity for fear of being re-arrested.Boko Haram's attacks have spilled into neighbouring countries and displaced more than 2.4 million people in the Lake Chad region, creating a vast humanitarian crisis. Some fighters have allied with the Islamic State group.While Nigeria's military has arrested many Boko Haram top fighters and late last year declared the extremist group had been "crushed", leader Abubakar Shekau remains elusive and the group in recent months has carried out a growing number of deadly suicide bombings and other attacks. Many have been carried out by women or children.