BANNU - Unidentified assailants on Wednesday gunned down a polio immunisation supervisor in the district, police said. Police said the polio supervisor, Rafidullah, son of Munawwar Khan, resident of Kurram Ghari village, was staying at a shop in Pipal bazaar when all of a sudden two motorcyclists came and opened fire at him, as a result of which the polio worker died on the spot. The attackers fled from the crime scene. They, however, left a mobile set and the motorcycle on the spot. Soon after the incident, police rushed to the site, cordoned off the area and started search operation to arrest the culprits. However, no arrest was made till filing of this report.