As U.S. military, Pakistani forces and Afghan officials sought to piece together the incident, Afghan, U.S. and Western officials said the attack took place in response to fire from the remote Pakistani posts in the Mohmand tribal region, a lawless border area that abuts Afghanistan's eastern Kunar province.

WSJ: Airstrike Ravages U.S.-Pakistan Ties

Hekmatyar's stomping grounds include Kunar province, a sweep of heavily forested mountains in eastern Afghanistan.

NPR: For Afghan Fighter the War Continues as Loyalties Shift

Scott Sentell, the U.S. brigade's executive officer, was one of the soldiers who tried in 2011 to transfer control of Camp Blessing in Kunar Province to Afghan forces.

WSJ: Role Reversal: Pakistan Fears