Iran says the misalignment of an air defense unit's radar system was the key "human error" that led to the accidental downing of a Ukrainian passenger plane in January.

"A failure occurred due to a human error in following the procedure" for aligning the radar, causing a "107-degree error" in the system, the Iranian Civil Aviation Organization (CAO) said in a report published late Saturday.

This mistake "initiated a hazard chain" that caused further errors in the minutes before the plane was shot down.

The CAO document was described as a "factual" account of what happened but is not the final report from the investigation.

'Accidental' downing killed 176

Flight 752, a Ukraine International Airlines jetliner, was struck by two missiles and crashed shortly after takeoff from Tehran on January 8. The downing came days after the US killing of Iran’s General Qassem Soleimani.

Iran took several days to admit that its forces had accidentally shot down the plane, killing all 176 people on board.

Tehran’s air defenses were on high alert at the time in case the US retaliated against Iranian strikes on US troops stationed in Iraq hours earlier. Those attacks were in response to the US killing of Soleimani.

Read more: Opinion: Protests and mourning in Tehran

Defense operator's error

The CAO said that the defense unit operator could have still determined his target was an airliner despite the erroneous radar information, but there was a "wrong identification". 

The operator launched two missiles at the aircraft "without receiving any response from the Coordination Center," according to the report.

The second missile, fired 30 seconds after the first, found its target "by observing the continuity of trajectory of the detected target."

ed/mm (AP, AFP)