KABUL, Afghanistan (CNN) -- A U.S. military probe has found that an airstrike in western Afghanistan killed at least 33 civilians last month -- in sharp contrast to the five to seven civilian deaths initially reported.
The August 22 airstrike in the Shindand District of the western Afghan province of Herat also killed 22 militants, said the report to Lt. Gen. Martin E. Dempsey, acting commander of the U.S. Central Command.
"This investigation was comprehensive and included independent information from witnesses in Azizabad and from previous investigations," the report said.
"Additionally, the investigating officer found that while there were increased numbers of civilian casualties, coalition forces acted based on credible intelligence, in self-defense, and in accordance with the Standing Rules of Engagement and the law of war."
The investigation, which does not assign blame for the civilian deaths, focuses on why the military initially reported that only five to seven civilians were killed, the official said.
The military stood by that death toll despite assessments from the Afghanistan government and the United Nations Assistance Mission in Afghanistan concluding that 90 civilians were killed -- most of them women and children.
But when cell phone pictures were later provided to the U.S. military showing dozens of bodies at the scene of the strike, the top U.S. military commander in Afghanistan, Gen. David McKiernan, asked U.S. Central Command to review the initial investigation.
The case sharpened tensions between the Afghan government and the United States over rising civilian casualties.
U.S.-led coalition forces have been battling a resurgent Taliban in Afghanistan, and they have stepped up airstrikes on suspected insurgent strongholds.
Weeks after the deadly strike, U.S. Secretary of Defense Robert Gates traveled to Kabul, where he met with President Hamid Karzai and publicly apologized for the incident.
"I offer all Afghans my sincere condolences and personal regrets for the recent loss of innocent life as a result of coalition air strikes," Gates said at the September 17 news conference outside the U.S. Embassy in Kabul.
"While no military has ever done more to prevent civilian casualties, it is clear that we have to work even harder," he said.
CNN Pentagon Correspondent Barbara Starr contributed to this report
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