COMBAT OUTPOST SANGESAR, Afghanistan — A burst of gunfire snapped First Sgt. Joseph Hissong awake. Then came another, and another, all with the familiar three-round bursts of an American assault rifle — and the unfamiliar sound of its rounds being fired in his direction.

The shooters were close. His first thought: “Are Taliban inside the wire?”

But it was not the Taliban. Over the next 52 minutes, as his company of paratroopers braved bullets and rocket-propelled grenades in the predawn darkness to retake one of their own guard towers in southern Afghanistan, they found themselves facing what has become a more pernicious threat: the Afghan soldiers who live and fight alongside the Americans.

The attack on Sergeant Hissong’s company, on March 1 at Combat Outpost Sangesar, left two Americans dead along with two Afghan assailants, but it was not the first time that Afghan solders had attacked forces from the American-led coalition, nor would it be the last of what the military calls “green on blue” attacks. Already this year, 22 coalition service members have been killed by men in Afghan uniform, compared with 35 for all of last year, according to coalition officials.

Yet with the coalition as a matter of policy offering only the barest of details about the attacks — the episode at Sangesar, for instance, was disclosed in a 71-word coalition statement — interviews conducted during a week at this outpost provided a rare and detailed account of the violence.