The Revolutionary Armed Forces of Colombia, FARC have blamed negligence for the death of Roberto Sepulveda Muñoz, a former FARC member who had been imprisoned for nine years.
Muñoz was set to be released under the Amnesty Law but died at a hospítal after his health deteriorated.
The former FARC member had been suffering from a terminal illness.
The FARC argue Muñoz did not receive adequate medical attention. According to a statement, Muñoz's disease was not treated efficiently and became chronic bronchopneumonia.
The statement demanded a full and "thorough investigation" be carried out to determine the cause of Muñoz's death.
"Despite numerous humanitarian requests for his release, nine months after an Amnesty Law was passed, two regulatory decrees, an administrative resolution appointing him a peace manager and multiple commitments from the national government could not grant him release," the FARC document said.
The letter also urged the local and federal authorities to guarantee medical care for "all the FARC members who are still in detention" and will be released definitively.
"We call on the national government to establish responsibilities and to definitively release our comrades who are still in prison," the FARC statement concluded.
The Colombian government and the FARC signed a peace agreement in November last year but the FARC argue the government has failed to properly protect former guerrilla members.