Eleven of the herders were said to have been killed in Yagba west while the other four were killed in Dekina.
Joseph Olutimi, administrator of Yagba west said the killings occurred between May 18 and May 20.
Olutimi said the initial report he got was that a farmer was killed by herdsmen, saying that he quickly informed security agents of the development.
Jerry Omadara, special adviser on security to Bello, said the herdsmen were killed as a reprisal for the death of the farmer.
He also said three corpses of herdsmen were found in Oluja community in Dekina, while several cows were also killed by unknown gunmen.
The governor ordered four local administrators in the state to produce the perpetrators of the crime within seven days.
Bello issued the ultimatum to the local government chiefs during a stakeholders meeting on security held in Lokoja, the state capital on Tuesday.
He said failure to produce the perpetrators would lead to the sack of the administrators who he also accused of ineptitude and dereliction of duty.
The affected are the administrators of Yagba West, Omala, Dekina and Igalamela local government councils.
The governor said his administration would not condone killings of any citizen in the state under whatever guise.
“This Fulanis have been staying with us in the state peacefully without any rancour, any politicians or detractors who are behind this killing will be made to face the full wrath of the law,” Bello said.
“My administration will not spare anybody no matter how highly placed in the society.”