Vadym Prystaiko, Ukraine's foreign minister, said on Friday evening that Kiev would reserve judgement about what happened until the flight recorders had been analysed and that it would not discard any version of what happened.
"Our task is to determine all the facts of the tragedy," he said. "We are looking at all possible explanations."
Some confusion remained as to what Mr Prystaiko meant by "access". He later suggested that Ukraine's team of investigators were yet to be given access to the data contained in the black boxes. Ukraine is pushing to get the flight recorders analysed in their country, he said.
Tehran has said it will process the information from the black box within Iran, inviting Boeing and Ukrainian officials to participate in the investigation. Concerns have been raised, however, about whether Iran has the capacity to process the information.
Ukraine has so far toed a diplomatic line of not being drawn on western intelligence assessments and a growing body of evidence that the the plane was accidentally shot down by Iranian surface-to-air missiles. Earlier in the day, Ukrainian diplomats confirmed they had received "very important data" from the US that they would be feeding into the investigation.
That announcement followed a remarkable plea for intelligence-sharing by the Ukraine's president, Volodymyr Zelensky, who suggested he had not been properly briefed by the Americans.
Writing on Facebook, Mr Zelensky said that the possibility that the flight had been shot down “cannot be ruled out but is not currently confirmed”.
Mr Prystaiko insisted that the 50-strong Ukrainian investigation team were enjoying the "full cooperation" of Iranian authorities, including access to flight paths and conversations between the pilots and air-traffic control. These were completely normal up until the crash, he added, with the plane flying within its assigned flight corridor.
Ivan Bakanov, head of Ukraine's security agency, said Kiev was prioritising two possible causes of the plane crash: a missile or terrorism.
Ukrainian International Airlines flight 752, a Boeing 737-800, came down near the Iranian capital shortly after take-off and a few hours after Tehran attacked US military targets in Iraq in retaliation for the assassination of Iranian general Qassem Soleimani. At the time of the crash, Iran was on full alert and anticipating an aerial response. All 176 people onboard the plane died.
Ali Abedzadeh, head of Iran’s Civil Aviation Organisation, echoed Ukraine's calls for intelligence to be shared with the investigating commission.
Speaking at a press conference on Friday morning, the aviation official confirmed that the accident took place at 8,000 feet, but he categorically ruled out a missile strike. That, he said, was "technically impossible" and incompatible with the pilot’s apparent attempts to turn the stricken plane back to the airport.
From a very early stage, Tehran has insisted that the crash was caused by mechanical failure. This inflexible stance appeared to undermine the prospects of a transparent investigation.
They were damaged further by reports that Iranian authorities had removed debris from the crash site and, according to one well-placed Ukrainian journalist, were withholding crucial evidence from Kiev’s investigators.
Even before the intervention of western intelligence, much of the publicly available evidence contradicted the claims of an engine malfunction.
According to flight data available online, the plane took off normally, but approximately two minutes into the flight stopped transmitting all data. This was consistent with a catastrophic event and engine failure would not usually be considered. The plane was almost new, had undergone scheduled maintenance just two days earlier, and there were no distress signals.
Within hours of the crash, images began circulating that purported to show the heads of Russian-made Tor surface-to-air missiles near the area where the plane crashed. However, the images were not verified.
There are conflicting reports that more than two anti-aircraft projectiles may have been fired at the plane.
A resident of Parand, a town near the airport, who spoke to a soldier posted in a nearby garrison, said they believed three missiles may have been fired.
“Based on the research that I have done, I understood that the base that is near us was on alert, they saw the aeroplane and fired at it,” the resident told The Independent.
“The soldier said the first rocket exploded before impact, the aircraft’s fuel tanks were pierced, and the other two rockets fired caused the plane to catch fire and then crash.”
BBC Persian correspondent Jiyar Gol told the BBC World Service on Friday that he was told by another witness, also a soldier in a nearby barracks, that he saw three missiles fired at the plane, but that only one hit.
The Independent could not independently verify the claims.
Speaking about why the pilot did not send a distress call, Mr Abedzadeh suggested that the pilot was concentrated on “saving the plane”.
“After take-off, after a few minutes the pilot contacted the tower and had asked permission for 26,000 feet, so this has all happened within two minutes,” he said.
Overnight, US media quoted intelligence sources offering a very different interpretation. According to CNN, the plane was hit by not one but two Iranian missiles, with US satellites tracking their infrared signatures.
CNN claimed that US authorities did not immediately go public with the data because they had decided to “verify” the data.
News broadcaster CBS cited US federal sources as saying it was likely a SA-15 air-defence missile system, also known as Tor, that took down the plane. The medium-altitude short-range surface-to-air missile has a maximum range of 20,000 feet.
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