WASHINGTON : A “friendly fire” incident in Afghanistan that killed five US troops and one Afghan could have been avoided if American forces had communicated properly and understood their aircraft’s capabilities, military investigators said Thursday.

US troops coordinating an air strike on June 9 failed to follow standard tactics when a B-1B bomber dropped two bombs on a ridgeline in southern Afghanistan, killing five Americans and an Afghan soldier, according to the US military probe.

The US and Afghan troops had called for air power after coming under fire in Zabul province but a group of soldiers that had moved to higher ground were mistaken for insurgents, according to the findings issued by US Central Command, which oversees forces in Afghanistan.

The B-1B bomber had been called in to offer “close air support” at the end of a joint US-Afghan operation designed to “disrupt” insurgent forces in Arghandab district before Afghan presidential run-off elections, the report said.

As the troops came under fire, several soldiers climbed to higher ground to outmaneuver the insurgents.

“This movement of friendly forces was not effectively communicated” to officers overseeing the air strike, “which led to the team incorrectly identifying muzzle flashes on the ridgeline as enemy activity - an incorrect target identification that was accepted by the aircrew,” it said.

In addition, the troops involved in the air strike “incorrectly believed” that sensors on the bomber could detect infrared marking devices that are supposed to identify friendly forces.

“While this complex combat situation presented a challenging set of circumstances, had the team understood their system’s capabilities, executed standard tactics, techniques and procedures and communicated effectively, this tragic incident was avoidable,” the report said.

Parts of the probe’s findings were censored.

Despite the loss of US and Afghan troops, the operation “disrupted insurgent activity” and bolstered security before the Afghan runoff elections, said the probe, which was led by Air Force Major General Jeffrey Harrigian.