Iran has refused to release the black boxes from a passenger jet that crashed shortly after take-off killing 176.
The two flight recorders could reveal why the airliner came down in flames so soon after Iran’s ballistic missile strike against US troops.
Mystery has surrounded the disaster as the Boeing 737 was seen in unverified video burning while in mid-air after taking off from Iran’s main Imam Khomeini airport.
There were 167 passengers and nine crew members on the Ukraine International Airlines flight PS752 to Kiev when it crashed.
The plane’s black boxes have been recovered, but the head of civil aviation in Tehran, Ali Abedzadeh, refused to hand them over to Boeing.
The Ukrainian embassy in Iran initially issued a statement saying: “According to preliminary data, the plane crashed due to engine failure for technical reasons. As of now, versions of terrorist attack or missile attack are ruled out.” However, that claim was later deleted from the embassy’s website.
Any previous comments about the cause were “not official”, the embassy said.
A video on state media appeared to show the plane already on fire as it fell from the night sky.
Asked if the airliner could have been downed by a missile, Ukraine’s Prime Minister Oleksiy Honcharuk cautioned against speculation.
But David Learmount, of news site FlightGlobal, said: “Whatever it was, it was very dramatic and very sudden, because the crew were not able to make an emergency call.
“They had plenty of height which would have given them the time to make a call. You try to work out what very dramatic thing can go wrong with an airliner...”
The ex-RAF pilot added: “If it was a missile strike, that’s dramatic. If it was a bomb on board, that’s dramatic.”
Mr Learmount said the crash could have been caused by the “catastrophic mechanical failure” of an engine, however he said this would not match the significant wreckage that has been reported by the Iranian media.
The plane was just three years old and passed its last technical check on January 6. Ukraine Airlines said the three pilots on board had between 7,600 and 12,000 hours’ experience flying a Boeing 737 aircraft.