ABUJA (Reuters) - The trial of more than 1,600 people suspected of ties with Boko Haram was expected to begin in Nigeria on Monday behind closed doors, in the biggest legal investigation into the eight-year militant Islamist insurgency. More than 20,000 people have been killed and two million forced from their homes in northeastern Nigeria during the insurgency, contributing to what the United Nations has said is among the world s worst humanitarian crises.  Nigeria s ministry of justice said last month the trial of around 1,670 people held at the Kainji detention facility would begin at the site, in the central Niger state, on Monday and would be presided over by four judges.  A spokesman for the ministry did not respond to requests for confirmation that the trial had begun. A military spokesman declined to comment, saying questions should be addressed to the judiciary.  The ministry has said that after the Kainji trials are completed, a further 651 people suspected of having links to Boko Haram and currently being held at prisons in Maiduguri, the capital of the northeastern state of Borno, would go on trial.   Clement Nwankwo, a human rights lawyer based in the capital, Abuja, said the trials would provide a more effective deterrent if they were open to the media and public.  On the Boko Haram issue, stories need to be told for the public to be made aware what has been going on and understand the nature of the crimes committed,  said Nwankwo, adding that secrecy also made it hard to determine whether trials were fair.   The Nigerian authorities have not been known to be diligent in investigating and properly prosecuting suspects,  he said, warning that a sense of injustice could breed resentment among relatives that could yield future radicalization.  However, Fatima Akilu - who headed the government s counter violent extremism program under the previous administration - said secrecy was needed to encourage witnesses and judges to take part in the trials because Nigeria does not have a witness protection program.  A lot of witnesses were afraid to come forward,  Akilu, who was based in the Office of the National Security Adviser from 2012 to 2015, said of previous efforts to pursue trials.  She said judges and witnesses had previously been subjected to death threats.   If the witnesses don t come forward there is limited evidence in terms of reaching a conviction, so I think there was little choice,  she said, adding that there were no clear alternatives in the absence of an amnesty program. Nigeria s handling of thousands of people accused of ties with Boko Haram insurgents has previously attracted criticism.  The legal process marks a steep escalation in the number of insurgency-related cases being handled by Nigerian authorities. The Ministry of Justice has said that, as of Sept. 11, only 13  terrorism cases  had been concluded and nine convictions had been secured.   The decision to start the trials is a response to persistent complaints by local and international human rights groups over thousands of persons detained without access to lawyers and without any specific charges, said Nnamdi Obasi, of International Crisis Group.  