The "truck" used in the transfer had room for 40 people but was not equipped with individual cells for each inmate.
Four inmates were killed during a deadly prison fight in the northern Brazilian state of Para while being transferred to another penitentiary, officials said Wednesday.
A vehicle carrying 30 handcuffed inmates inside four compartments left the Regional Recuperation Center in the Amazonian city of Altamira on Tuesday afternoon on a 500-kilometer trip to a prison in the city of Maraba.
Four of the prisoners, allegedly all belonging to the criminal gang accused of sparking the fight, were killed by asphyxiation during the transfer, according to a statement by Para's Public Safety and Social Defense Secretariat.
Authorities added they became aware of the tragedy after the vehicle had arrived in Maraba.
The "truck" used in the transfer had room for 40 people but was not equipped with individual cells for each inmate, the statement said, adding that the Para government does not have access to such a vehicle.
Upon arrival, the other 26 transported prisoners were housed in solitary cells at the Maraba penitentiary.
The secretariat said an investigation was opened.
A total of 58 people were killed in a brutal fight between rival gangs on Monday inside the Regional Recuperation Center in Altamira.
The bloody clash ended with 16 inmates decapitated and 42 others dead of smoke inhalation after one of the gangs set fire to a cell block.
Afterward, the Para government arranged to transfer 46 inmates purportedly involved in the massacre to other penitentiaries in that state.
Acting on a request by Gov. Helder Barbalho, Brazilian Justice Minister Sergio Moro also has authorized the deployment of federal police to Para prisons for 30 days.
Following the massacre in Altamira, the deadliest of 2019, more than 100 inmates have now been killed this year at prisons in northern Brazil in clashes involving criminal gangs fighting for control of lucrative Amazonian drug-trafficking routes.
In May of this year, 55 inmates were killed in a span of less than 48 hours at four penitentiaries in Manaus, the capital of Amazonas state.
Considered one of the worst episodes in Brazil's correctional system since 1992, when military police killed 111 inmates following a riot at the Carandiru Penitentiary in Sao Paulo, the Para massacre has reopened a debate in the country about violent and overcrowded prisons.
Though the prison service says the Altamira penitentiary was built to hold 200 prisoners and was housing 311 at the time the gang fight erupted, a report from the National Council of Justice indicated the facility was in fact holding 343 prisoners - more than double its capacity of 163.