Kabul, Afghanistan CNN — Five American service members were killed in southern Afghanistan when a coalition jet, called in to help ward off a Taliban attack, mistakenly bombed them, an Afghan official said Tuesday.The five were killed Monday night along with an Afghan soldier in Zabul province, said Ghulam Sakhi Roghliwanai, the province’s police chief.According to NATO, the troops were conducting a security operation. Such operations have been stepped up ahead of Afghanistan’s presidential runoff election, which will take place Saturday.The service members’ unit came in contact with enemy forces. That’s when the casualties occurred, a U.S. military official told CNN. “There is the possibility that fratricide may have been involved,” he said. The incident is under investigation.A NATO statement provided similar information.Roghliwanai said the troops had completed their joint military sweep at about 9 p.m. Monday 12:30 p.m. ET when they came under rocket fire from Taliban militants.