On Wednesday, the Department of Defense identified the soldiers from JBLM as Sgt. Sapuro B. Nena, 25, Pfc. Genaro Bedoy, 20, and Pfc. Jon R. Townsend, 19. All three were assigned to the 3rd Stryker Brigade Combat Team, 2nd Infantry Division.
The fourth soldier, identified as 22-year-old Spc. Joshua N. Nelson, was assigned to the 202nd Military Intelligence Battalion, 513th Military Intelligence Brigade based in Fort Gordon, Ga.
Bedoy, Townsend and Nelson were on their first deployments. Nena was deployed in Iraq in 2009-2010.
Afghan officials said the checkpoint in Zabul province's Mizan district came under attack first from insurgents sometime around midnight. American forces came to help the Afghan police respond to the attack, said Ghulam Gilani, the deputy police chief of the province.
It was not clear if some of the Afghan police turned on their American helpers in the middle of the battle with the insurgents, or afterward, or were somehow forced into attacking the American troops by the insurgents, Gilani said.
One police officer was killed in the clash with NATO troops, but other officers at the site fled and it was unclear if they were involved in the attack, said Jamie Graybeal, a spokesman for the international military in Afghanistan.
The killings prompted NATO to change how the U.S.-led coalition will fight the war in Afghanistan.
Previously, coalition troops routinely conducted operations such as patrolling or manning outposts with small units of their Afghan counterparts. Under the new rules issued on Sunday, such operations with small-sized units are considered no longer routine and require the approval of the regional commander.
NATO's decision reflected escalating worries about the insider attacks, coupled with the widespread tensions over an anti-Islam video that has prompted protests around the world, including Afghanistan.
So far this year, 51 international service members have died at the hands of Afghan forces or militants wearing their uniforms. That is more than 18 percent of the 279 international troops who have been killed in Afghanistan since the beginning of the year, according to figures compiled by The Associated Press.