Iran said the misalignment of an air defence 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 Organisation (CAO) said in a Saturday (July 11th) report.
This error "initiated a hazard chain" that saw further mistakes committed in the minutes before the plane was shot down, said the CAO document, presented as a "factual report" and not as the final report on the accident investigation.
A high ranking Ukrainian official, who asked to remain anonymous, said the latest announcement "is not particularly credible" because Iran had given many different versions of the story and changed them too often.
Flight 752, a Ukraine International Airlines jetliner, was struck by two missiles and crashed shortly after taking off from Tehran's main airport on January 8th, at a time of heightened US-Iranian tensions.
The Islamic republic admitted several days later that its forces accidentally shot down the Kiev-bound plane, killing all 176 people on board.
The majority of the passengers were dual national Iranian-Canadians but they also included Ukrainians, Afghans, Britons and Swedes.
The CAO said that, despite the erroneous information available to the radar system operator on the aircraft's trajectory, he could have identified it as an airliner, but instead there was a "wrong identification".
The report also noted that the first of the two missiles launched at the aircraft was fired by a defence unit operator who had acted "without receiving any response from the Co-ordination Centre" on which he depended.
The second missile was fired 30 seconds later, "by observing the continuity of (the) trajectory of the detected target", the report added.
The CAO said there was a defect in the transmission to the defence units' co-ordination centre of the data identified by the radar.
An Iranian general had said in January that many communications had been jammed the night of the disaster.
Tehran's air defences had been on high alert at the time the jet was shot down in case the US retaliated against Iranian strikes hours earlier on US troops stationed in Iraq.
Those strikes were carried out in response to the killing of top Iranian general Qassem Soleimani, in a US drone attack near Baghdad airport.
The aircraft tragedy sparked fierce reprobation in Iran, especially after it took three days for the armed forces to admit having shot down the plane "by accident" after a missile operator mistook it for an enemy projectile.
Ottawa and Kiev have demanded for months that Iran, which does not have the technical means to decode the black boxes, send them abroad so their contents can be analysed.
Canada's Foreign Minister François-Philippe Champagne on Sunday urged Iran to ensure "a comprehensive and transparent investigation in accordance with international standards, so that all those responsible are held accountable".
In late June, France's Accident Investigation Bureau (BEA) said Iran had "officially requested technical assistance" to retrieve the black box data and said work should begin on July 20th.
Champagne said Canada would continue to push "to ensure Iran follows through on its commitments" including transferring the data to the BEA.
In early July, Canada announced that it had reached an agreement in principle with Iran to launch negotiations on compensation for the families of foreign victims of the accident.