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.
Pentagon spokesman Rear Adm. John Kirby acknowledged the deaths appear to be the result of friendly fire.
"I'm not going to get into too much detail about qualifying who they were or the specifics of the mission," Kirby said. "We do have reason to suspect that friendly fire was the cause here, specifically friendly fire from the air, but the issue is under investigation."
A NATO statement provided a few details.
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.
The service members called in air support. "But the airstrike mistakenly bombed their own friends too," he said.
"The Department of Defense is looking into what happened," U.S. National Security Council spokeswoman Caitlin Hayden said. "Our hearts are heavy over this loss, and our thoughts and prayers are with the families of the fallen."
Friendly fire incidents
Since the Afghan war began in October 2001, 2,323 Americans have died there. Once the United States confirms these latest deaths, the number will rise to 2,328.
There have also been numerous fatalities resulting from friendly fire.
Among the more publicized ones was the 2004 death of Pat Tillman. Tillman, who became a national hero after he gave up a lucrative contract with the NFL's Arizona Cardinals to join the Army's elite Rangers force, was awarded the Silver Star, the military's third-highest combat decoration, after the Army said he was killed leading a counterattack.
Only later did the Army admit he had been shot accidentally by his comrades.
Here are some of the others:
• In April 2002, four Canadian service members were killed and eight others seriously wounded when a U.S. Air National Guard pilot dropped a 500-pound bomb on a group of Canadian troops conducting a nighttime training exercise in southern Afghanistan.
• In August 2007, three British soldiers were killed when a U.S. Air Force F-15 Eagle mistakenly struck their position with a 500-pound bomb during a firefight with Taliban fighters in northern Helmand province.
• In July 2008, nine British soldiers were wounded when a UK Apache attack helicopter mistakenly fired on them during a firefight with Taliban fighters in central Afghanistan.
• In November 2011, NATO helicopters attacked a military checkpoint in northwest Pakistan, killing 24 Pakistani soldiers. The attack wounded 13 others in the volatile region bordering Afghanistan. The incident greatly strained relations between the nations.
The Taliban abducted a busload of students and lecturers from Kandahar University as they traveled to the capital, Kabul, on Tuesday, said Mohammad Ali Ahmadi, the deputy governor of Ghazni province.
The exact number of passengers on the bus was not immediately known.
Negotiations were under way to find the passengers and free them, Ahmadi said.
CNN's Masoud Popalzai reported from Kabul; Ed Payne reported and wrote from Atlanta. CNN's Ric Ward, Gabe LaMonica and Michael Martinez contributed to this report.