(CNN) -- A friendly fire incident that killed six soldiers in Afghanistan, including five Americans, was a result of botched communication and a series of other errors, the Pentagon said.
The June 9 bombing in Zabul province is among the deadliest friendly fire incidents in the 13-year war. It also left one Afghan soldier dead.
What started as a battle with insurgents on a hillside descended into a tragedy after failures in equipment and communication protocol, according to a Pentagon report released Thursday.
Teams involved did not follow established procedures, and one radio had no batteries, while another had a broken talk button, according to the report.
'A challenging set of circumstances'
In addition, soldiers didn't relay their position correctly, and a supersonic strategic bomber wrongfully relied on a sensor to detect soldiers.
"Though this was a challenging set of circumstances, had the team executed standard tactics, techniques and procedures, and communicated effectively, this tragic incident was avoidable," Air Force Maj. Gen. Jeffrey Harrigian said in the report.
The 300-page report by U.S. Central Command was partially redacted before its release under the Freedom of Information Act.
"The key members executing the close air-support mission collectively failed to effectively execute the fundamentals, which resulted in poor situational awareness and improper target identification," Harrigian said.
At the time, U.S. and Afghan forces were using a supersonic strategic bomber to target insurgents and ensure security at polling places five days before the June 14 presidential runoff election.
Bomber mistakes soldiers for enemy
To tackle fire better from insurgents, a team on the ground split up at the request of Staff Sgt. Jason A. McDonald, the report said.
"Communication degradation due to terrain" made it difficult for McDonald to tell the crew of a B1-B Lancer in the area that the team had split up and moved to higher ground.
McDonald could not contact the team moving to the hillside because one member had a radio that was out of batteries and the other carried one with a broken "push-to-talk" function. McDonald then sent Spc. Justin R. Helton up the hill to act as a verbal communications link.
Meanwhile, at 12,000 feet, a pilot flying the B1-B Lancer scanned the area with night-vision goggles, looking for infrared strobes worn by soldiers to broadcast their presence.
All the pilot saw were occasional muzzle flashes, which he thought were from the enemy, the report said.
Sensors on the B1-B don't detect infrared strobes, and the night-vision goggles the pilot used are not designed to see infrared strobes at an altitude higher than 7,000 feet.
The bomber did not have permission to fly under 12,000 feet because of the presence of U.S. drone aircraft at lower altitudes.
About 7:30 p.m. on June 9, the B1-B dropped the two bombs that killed the soldiers.
Staff Sgt. Scott R. Studenmund, 24, of Pasadena, California, was wearing the strobe on the back of his helmet when he was killed. McDonald, 28, of Butler, Georgia, who split the hillside team, was also killed.
The other U.S. deaths included Helton, 25, of Beaver, Ohio; Cpl. Justin R. Clouse, 22, of Sprague, Washington; and Pvt. 2nd Class Aaron S. Toppen, 19, of Mokena, Illinois.