"Around evening time on Monday, we saw some people in a bus approach the police station, shoot the gate open, and killing all the six officials who tried to resist them," said Festus Okoye.
The $1.6-billion (1.34-billion-euro) Chinese-built link between Nigeria's economic hub Lagos and Ibadan, capital of the southwest region, began partial operations last December.
The state has been the hardest hit by bandits despite a government offer of amnesty in exchange for renouncing violence and surrendering their weapons.
Since their release, the students have been in government care in state capital Gusau, where they are receiving medical treatment following their ordeal in captivity.