First Sgt. Joseph Hissong told The New York Times two members of his unit died in that attack.
The insider attack is "a game-changer on all levels," Hissong said.
A surge in insider attacks, in which people turn on others they are supposedly helping, offers a clear sign Afghan resentment of involvement by foreigners is becoming unmanageable, officials said.
U.S. officials are concerned about the attacks' disruptive effect on the training mission at the core of the U.S. combat troop withdrawal plan for 2014.
Cultural clashes have contributed to some of the insider attacks as has abusive behavior by Afghan officers seen as backed by Americans, officials said.
Many senior coalition and Afghan officials have concluded that, after nearly 12 years of war, the view of foreigners held by many Afghans now mirrors that of the Taliban, turning hope to hatred and prompting young men to suggest insider attacks to Taliban leaders.
One coalition officer said a high percentage of the insider attacks "have the enemy narrative -- the narrative that the infidels have to be driven out -- somewhere inside of them, but they aren't directed by the enemy."