Javid Faisal, a spokesman for the governor of Kandahar province, said the killings occurred at a post in Spin Boldak district, Khaama Press reported. He said the suspected killer got away and had not been apprehended.
Three officers who survived the shooting were hospitalized with injuries, Faisal said.
Neither the Taliban nor anyone else had claimed responsibility for the killings.
A Pentagon report to Congress, released last week, says most insider attacks by Afghan security forces on coalition troops were likely triggered by personal motives, the Afghan online news service Khaama Press reported.
Military investigators found that, of 79 reported insider attacks from May 2007 to September 2012, 38 percent were likely to have been triggered by personal motives, also known as "green on blue" attacks; 6 percent were suspected cases of enemy infiltration and 14 percent were attributed to co-option, in which insurgents persuade a security force member to assist in an attack.
The causes of the remaining 38 percent of the cases are unknown or still under investigation, the report said.
U.S. officials say they remain focused on enhancing training and security procedures.
"I would be very loath to say that it (inside attacks) is a problem that's been solved. It's a problem we will continue to address," a senior Defense Department official told The Washington Post.