Both sides said they believed they were attacking insurgents along the border. A senior Pakistani defense official acknowledged that Pakistani troops fired first, sending a flare, followed by mortar and machine-gun fire, toward what he said was “suspicious activity” in the brush-covered area below their high-altitude outpost barely 500 yards from the border…

Despite extensive coordination mechanisms set up to prevent such encounters, the U.S. military failed to respond to Pakistani alerts that its troops were being bombed, said the Pakistani defense official, who spoke on the condition of anonymity because he was not authorized to discuss the issue on the record.

“We told them, hold your horses, these are ours,” the official said. While repeated urgent appeals went up the coalition chain of command, he said, the airstrike continued for an hour and a half against two Pakistani border positions and a contingent of troops.

Administration and U.S. defense officials raised the possibility of a different set of circumstances but declined to elaborate.