Forty-four suspected Boko Haram terrorists were found dead in their cells in a Chad prison last week.
Officials said the 44 were among a group of 58 suspects captured during a major army operation around Lake Chad marshlands launched by President Idriss Deby early this month.
Youssouf Tom, the Chadian Public Prosecutor said: “We did a head count, we found that only 44 of 58 prisoners were actually called back to God, under circumstances we don’t know about.”
An autopsy carried out on four of the dead prisoners revealed traces of a lethal substance that had caused heart attacks or severe asphyxiation, the Public Prosecutor added.
“We have automatically decided to open a judicial inquiry to determine the cause of this mass death of inmates,” the prosecutor added.
The deaths by apparent poisoning were shrouded in mystery on Sunday, with observers wondering whether they were murdered or had committed collective suicide.
The semi-desert country lost 98 soldiers in a March 23 attack on a base at Bohoma in the Lake Chad marshlands, prompting a massive military operation against Boko Haram.