A US military investigation has concluded that a series of "unintentional human errors" led to a 17 September coalition air strike that killed fighters aligned with the Syrian government instead of the targeted Daesh militants.
The strike, which Moscow said killed more than 60 Syrian soldiers, prompted an emergency United Nations Security Council meeting as tensions between Russia and the United States spiked.
Brigadier General Richard Coe, who led the investigation, told reporters at the Pentagon on a conference call on Tuesday that the major errors ranged from a basic misidentification of targets to "group think" during intelligence development and even a communications blunder on a hotline with Russia.
Coe also defended the coalition personnel involved, saying they were "good people trying to do the right thing."
"These people get it right far more often than not, but this time they came up short," Coe said.
The investigation threw light on the difficult and dangerous work of developing targets for coalition air strikes against Daesh in parts of Iraq and Syria where the United States does not have ground forces or reliable informants within the population to ensure its intelligence is sound.
The US-led coalition mistook Syrian-aligned forces for Daesh fighters in part because they were not wearing traditional uniforms, Coe said. An early mistake – misidentifying a vehicle as belonging to Daesh – coloured intelligence that came later when it drove into a larger fighting position near Deir al-Zor airport.
Coe acknowledged that major red flags were missed. One analyst saw a tank moving around and even typed into a network chat room that "what we are looking at can't possibly be ISIL," Coe said, using an acronym for Daesh.
The strikes included aircraft from the United States, United Kingdom, Australia and Denmark, which dropped 34 precision-guided weapons and fired 380 rounds of 30-millimeter ammunition.
There was no evidence of deliberate disregard for targeting rules, Australia's chief of the Defence Force, Air Chief Marshal Mark Binskin, said in a statement on Wednesday.
"In many ways these forces looked and acted like the Daesh fighters the Coalition has been targeting for two years," he said, referring to Islamic State by one of its Arab acronyms.
The mistakes continued even after the strike began.
Moscow had reached out repeatedly through a hotline to the US-led coalition, trying to inform them that they were striking Syrian regime targets instead of Daesh.
But the designated US military point-of-contact was unavailable for 27 minutes. In those 27 minutes, 15 of the strikes took place against what the US-led coalition believed were Daesh fighters.
"This was obviously a missed opportunity to be able to limit the damage of the mistake," Coe said, adding that the strikes would have continued had the Russians not called and eventually passed along their information.
Even before the strike took place, the coalition made a big mistake. It initially contacted the Russians to inform them that aircraft would be near Deir al-Zor, but gave the wrong coordinates for the strike, Coe said.
"Of course we don't know for sure, but it is possible had we passed the right location to the Russians, they would have had the opportunity to warn us before the first strike even started," Coe said.
No disciplinary action is expected against those involved, a decision Pentagon spokesman Peter Cook defended on Tuesday.
"As we understand it, based on the investigation, there was no malice here..This is what they believed to be an appropriate target," Cook said during a news briefing.