The Army investigator called this circumstance ‘‘worth noting,’’ but he established no proof of complicity by the Afghan security officers.

An Afghan investigation concluded that only one soldier, a sergeant identified as Enayut (Afghans often use just one name) fired on the Americans, according to a summary of the probe, while the U.S. Army concluded there were two shooters.

Several U.S. soldiers recalled noticing two, possibly three, Afghans enter the compound about 9 p.m. They stood out because they were armed with one rocket-propelled grenade and at least one M16 rifle. At least one was wearing an Afghan army uniform, the report said. No one questioned them, since there was no screening requirement in place.

‘‘They just walked in like they owned the place,’’ a U.S. sentry at the compound’s barricaded entrance told investigators afterward. Like others, his name was blacked out of the report.

In the moments that followed, hints of trouble were obscured by the appearance of normalcy.

At 9:02 p.m., just a few minutes after taking up his guard position at the front entrance of the command post tent, Spc. Paul A. LeVan was told he was being repositioned to a guard tower overlooking the compound. He was not replaced at the tent. There was no explanation as to why.

LeVan’s sergeant led him to the empty guard tower, where, as a standard precaution, they discussed the locations of friendly forces in LeVan’s line of fire. He was armed with an M249 light machine gun.

Soon, two of the Afghans who had entered the compound at 9 p.m. joined them in the tower. One was in military garb and, rather curiously, armed with a grenade launcher and one grenade. The other was unarmed and spoke English. LeVan’s sergeant then left the tower and, upon entering the command tent, mentioned the grenade launcher to those inside, including an enlisted soldier who recalled later that the weapon seemed ‘‘out of the ordinary.’’

‘‘But since (Afghan soldiers) were allowed to carry RPGs (rocket-propelled grenades), we did not give it much thought,’’ the enlisted soldier, whose name was blacked out of the report, told investigators.

Another unidentified soldier said in the report that he, too, noticed the RPG and thought it ‘‘seemed reckless’’ to permit it inside the compound.

In his final report, the Army investigator found it curious that neither LeVan nor his sergeant challenged or questioned the two Afghans about ‘‘why a tower guard would have an RPG and no rifle.’’

LeVan, 21 at the time and a member of the 209th Military Police Company, said he assumed the Afghans were a properly assigned guard and his interpreter, although he noticed that the armed Afghan was avoiding eye contact and closely tracking movements inside the compound. LeVan shook hands with both men, but the veneer of friendliness soon vanished.

‘‘I had a gut feeling that something was out of place,’’ he told the AP in a telephone interview. He was the only American to witness the attack from start to finish.

Suddenly and without explanation the Afghans descended from the tower.

‘‘I got nervous, so I kept a very close eye on the two men,’’ LeVan told an Army investigator two days later.

LeVan said he watched through his night vision goggles as the Afghan armed his grenade launcher and took aim at several Army medics playing cards on cots they had set up at the rear of their armored ambulance. A medic recalled spotting the gunman pointing the RPG at them from point blank range. ‘‘I stood up and shouted, ‘Hey! What the f--- are you doing?'’’ she told investigators.

His rocket missed the soldiers and slammed into a nearby concrete barrier. Shrapnel wounded the medic in her stomach and back. A piece of shrapnel also penetrated the nearby command tent, wounding the U.S. sergeant who had just left LeVan in the guard tower.

By several accounts, bullets began flying about five seconds after the grenade exploded.

‘‘The timing was perfect,’’ LeVan recalled. He watched from the tower as another gunman — not the one who launched the rocket, and not the English-speaking Afghan, either — advanced swiftly on the command tent, firing bursts from an M16 semi-automatic rifle.