Two people have been arrested in connection with the killing of the six aid workers, according to an official.
The attack happened in Maban, Upper Nile state, last month.
The County Commissioner says eye witnesses identified the two suspects and the authorities have arrested them.
“2 of those who have committed the crime have been arrested and under investigation,” Mr Timothy Mounthe confirmed it to Eye Radio.
Some humanitarian organizations had suspended operations in Maban County in fear of the situation.
He says a few of them have resumed operations, after discussions with the county authorities.