The children, aged nine to 12, were killed when a fighter-bomber strafed a mud-walled compound in Ghazni, central Afghanistan, in December. A prompt US apology was made.
The target, a suspected Taliban commander, was not killed, possibly because the wrong house was attacked.
A US spokesman in Kabul, Lieutenant Colonel Bryan Hilferty, said the investigation had found that the pilot followed "appropriate" rules of engagement. The report could not be made public, he said, given "the intelligence and the targeting involved". The UN condemned the killings at the time and has repeatedly called for a public inquiry.
"We did slightly change our rules of engagement after [the] investigation," Col Hilferty said, but these changes were also "top secret".
Some 200 tribal chiefs and elders have spent 10 days in Kabul complaining of US tactics in south-eastern Paktika. Their leader, Sher Khan, accused soldiers of detaining and beating innocent people. "We want US forces to coordinate with us before launching operations, otherwise the misunderstandings and mistrust will grow," he told Reuters.
Another chief, Haji Arsala, said the Americans had behaved worse than the Soviet Union's Red Army in the 80s: "They've not done a single bit of reconstruction work in our district. All they've done is give chewing gum to the kids."
