KABUL, Afghanistan — U.S. military investigators have concluded that the Chinook helicopter crash in Afghanistan that killed 30 U.S. troops in August was caused by an insurgent who fired a rocket-propelled grenade that struck the rear rotor of the aircraft.

It was deadliest single incident for U.S. forces in the decade-long war.

The Aug. 6 crash came amid rising fears that the country is far from stable even though U.S. and NATO forces have begun to leave Afghanistan. U.S. military officials have tried to counter those fears, saying that while the downing of the Chinook was a tragic setback, one crash will not determine the course of the war.

An official investigation report, issued Wednesday by the U.S. Central Command, said that after the grenade hit the rotor, the helicopter spun violently and then the main fuselage dropped vertically into a dry creek bed. The report said the copter was immediately engulfed in flames and the fire triggered several explosions of fuel and munitions.

No one survived the crash in Tangi Valley of Wardak province.

Among those killed were 17 members of the elite Navy SEALs, five Naval Special Warfare personnel who support the SEALs, three Air Force Special Operations personnel, an Army helicopter crew of five, seven Afghan commandos and an Afghan interpreter.