The US military continued to insist until September 7 that only seven civilians had been killed. They reconsidered their count when the video came out.
Why was the US count so low? According to the TimesOnline:
The US military said that its findings were corroborated by an independent journalist embedded with the US force. He was named as the Fox News correspondent Oliver North, who came to prominence in the 1980s Iran-Contra affair, when he was an army colonel.
Yes, Oliver North. Yes, that Oliver North.