KABUL, Afghanistan • U.S. military investigators have concluded that the Chinook helicopter crash in Afghanistan that killed 30 U.S. troops in August, including two Navy SEALs from Utah, was downed by a rocket-propelled grenade that hit the rear rotor. That caused the aircraft to fall vertically to the ground and burst into flames.
    It was deadliest single incident for U.S. forces in the decade-long war. The Taliban claimed responsibility for downing the helicopter; eight Afghan forces were also killed in the crash.
    The Utah SEALS killed that night were Jared William Day of Taylorsville, 28, and Jason Workman, 32, of Blanding. Both families declined to discuss their sons or their deaths.
    Day joined the Navy in 2002, and deployed many times to Africa, Asia and the Middle East, his family said in an August statement. Decorated many times with medals, he was an Information Systems Technician First Class and served as a tactical communicator for the elite Naval Special Warfare Development Group.
    Workman, a Special Warfare Operator 1st Class, was also highly decorated, with a Bronze Star for valor and a host of other medals. He was assigned to a SEAL unit on the East Coast. He joined the Navy in 2003.
    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 (http://tinyurl.com/3g4d4zt), issued Wednesday by the U.S. Central Command, said that after the rotor was hit, the helicopter spun violently and then crashed in a dry creek bed where it was engulfed in flames. The fire triggered several explosions of fuel and munitions.
    No one survived the crash in Tangi Valley of Wardak province, about 60 miles (97 kilometers) southwest of Kabul.
    Among those killed were 17 members of the elite Navy SEALs, five Naval Special Operations personnel who support the SEALs, three Air Force Special Tactics Airmen, an Army helicopter crew of five, seven Afghan commandos, an Afghan interpreter and a military dog.
    The report said that while final autopsy reports were still being reviewed, it’s believed that all 38 persons on board died rapidly after the crash.
    The troops killed were flying into the area to aid a U.S. Army Ranger platoon, which had been flown in earlier to try to kill or capture a Taliban leader.
    Investigators found no wrongdoing by personnel involved in the mission. The decision to transport the entire immediate reaction force in one helicopter was “tactically sound” to mitigate aircraft exposure to ground fire, the report said.
    However, it said the special operations task force commander did not reallocate intelligence, surveillance and reconnaissance aircraft to ensure ongoing surveillance coverage for both missions. While this was not the cause of the shoot-down or crash, the report said the issue should be addressed in similar missions in the future.
    The report dismissed speculation that the troops aboard the helicopter were lured into the valley by insurgents with advance knowledge of the landing site.
    “The shoot down was not the result of a baited ambush, but rather the result of the enemy being at a heightened state of alert due to three and one half hours of ongoing coalition air operations concentrated over the northwestern portion of the Tangi Valley,” the report said.
    According to the report, by 2:45 a.m. on Aug. 6, U.S. Army Rangers had cleared a compound and detained several suspected insurgents in Tangi Valley.
    Base commanders, however, were getting reports about insurgents assembling nearby and summoned the 17-member elite SEALs team.
    The group of insurgents was reported to be increasing in size so before the SEALs team left on its fatal mission, a decision was made to expand the squad to 33 personnel, including the Afghan interpreter and troops.
    Once it arrived near the compound, the helicopter descended to about 100 to 150 feet from the ground and slowed to about 58 mph.
    “A previously undetected group of suspected Taliban fighters fired two or three RPGs in rapid succession from the tower of a two-story mud brick building approximately 220 meters south” of the aircraft, the report said. “The first RPG missed the helicopter, but the second RPG struck one of the blades on the aft (rear) rotor assembly and exploded.”
    More than 10 feet of the rotor blade was lost and within seconds, the rear and the “forward rotor blade systems separated from the aircraft, and the main fuselage dropped vertically into a dry creek bed,” the report said.
    The aircraft was engulfed in a “large fireball” until it burned out several hours later.
    Eight hours later, the remains of all 38 victims and the military dog were recovered.
    The report also said the recovery efforts were complicated by a flash flood that filled the creek bed with 4 to 5 feet of water and washed parts of the wreckage up to 200 meters downstream. The wreckage recovery was completed three days after the crash.