Two Americans killed in unusual gunfight involving several Afghan soldiers In a war in which insider attacks have become commonplace, what happened next made the incident extraordinary, the American official said. Another Afghan soldier at the checkpoint opened fire on the Americans, killing a U.S. civilian contractor and wounding two other American soldiers. Soon, Afghan soldiers and possibly insurgents began firing at the Americans from several directions. NATO officials initially described the deadly episode as an insider attack but later said they had not yet classified it as such because of suspected insurgent involvement. A top Afghan military official denied that what took place was an insider attack and said the shooting was caused by a misunderstanding. A preliminary military report, however, has concluded that the gunfight began only after an Afghan soldier opened fire on U.S. troops, according to the American official, who spoke on the condition of anonymity. "What sets this apart is that there were multiple attackers from multiple positions and there was zero provocation," said the official, who had access to the report but was not authorized to speak for the record. Typically we are talking about a single gunman who acted in a somewhat rogue fashion, but in this case we are talking about an entire Afghan army unit and a large loss of life on both sides. Insider attacks have become the leading threat to the U.S. military's effort to wind down the war by the end of 2014, leaving behind a local force capable of protecting the fragile Afghan state. Before Saturday's incident, Afghan security forces had killed at least 51 NATO troops this year in 37 attacks, up from last year's 37 deaths in 21 attacks. U.S. military officials say roughly 25 percent of insider attacks have been linked to insurgent groups. They say a large number have stemmed from personal disputes. In unusually poignant remarks, the top U.S. commander in Afghanistan said over the weekend that he was "mad as hell" about insider attacks. "We're willing to sacrifice a lot for this campaign," Gen. John Allen said in an interview with "60 Minutes" that was broadcast Sunday. But we're not willing to be murdered for it. NATO officials issued a statement late Saturday describing that day's incident as an "apparent insider attack." The next day, NATO's deputy commander, Lt. Gen. Adrian J. Bradshaw, revised that assessment during a news conference in Kabul. "What was initially reported to have been a suspected insider attack is now understood to possibly have involved insurgent fire," he said, according to a transcript.