Complex sentence: After the attack, there was a controversy over the death of the 21 prisoners who died in the process of the attack, because some innocent prisoners might have been killed by the soldiers who labeled them as Boko haram.
Split into simpler sentences: After the attack, there was a controversies over the death of the 21 prisoners who died in the process of the attack. The basis of the controversies was on the fact that some innocent prisoners might have been killed by the soldiers who labeled them as Boko haram.