As investigators sift through the smoldering wreckage of a jetliner that crashed shortly after takeoff from Tehran, contradictory reports have emerged about what might have caused the Boeing 737-800 to abruptly plunge out of the sky.
Iranian authorities initially blamed "technical" causes then said an engine fire most likely led to the loss of the Ukraine International Airlines plane on Wednesday, killing all 176 people on board. The Ukrainian embassy in Tehran at first ruled out terrorism, then amended its statement to offer no comment on possible causes.
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Given that the crash of Flight 752 came hours after a barrage of Iranian missiles were fired on US bases in Iraq, speculation also turned to a possible accidental attack by Tehran's own air defence system.
The recently serviced, three-year-old jet went down without a distress call and after its global-positioning transmissions were cut off mid-air - unusual for a crash. A video purportedly shot by a bystander shows flames coming from the jet as it streaked across the night sky and burst into a fireball on impact.
Meanwhile, the investigation is being hampered by the international tension between Iran and the United States.
The US National Transportation Safety Board, which would normally be invited to participate in a crash of a US-built jetliner, issued a cautious statement saying it is monitoring the situation. Long-existing US sanctions against Iran require the NTSB to seek special permission to participate in any investigations in that country.
Iran's Disaster Mitigation and Management Organization said early assessments indicated the cause was a technical issue, while the transport ministry suggested an engine fire was involved.
A veteran aviation accident investigator said the flight-tracking data and amateur video were unlike a typical engine failure or fire scenario. While the evidence is still sparse, it's far more suggestive of a bomb or missile, said Jeffrey Guzzetti, former chief of the US Federal Aviation Administration's accident investigation division.
"Airplanes don't just catch fire and have that fire spread like that in such a short period of time, unless there was an intentional act causing that fire and explosion," Guzzetti said in an interview.
The tragedy recalled the 2014 downing of a Malaysia Airlines jet by a surface-to-air missile during the conflict over Crimea, eventually blamed by investigators on pro-Russia rebels. Complicating matters, Iranian officials haven't been clear about how much access US experts will be given to the probe, or to the 737's flight data recorders.
A state-run news outlet cited an official at Iran's Civil Aviation Organization as saying the pilots didn't declare an emergency.
If the amatuer video turns out to be genuine, it indicates the plane was essentially a fireball as it plunged, Guzzetti said. Bright lights emanating from the aircraft suggest explosions as well, he said.
Ukraine International said 167 passengers and nine crew were on the 737, an older version of Boeing's narrow-body workhorse that predates the Max model that was grounded last year following two fatal crashes.
The model involved in the Iranian crash doesn't use the flight control system implicated in the Max tragedies.
The Ukrainian jet lifted off and climbed to about 7,900 feet and was travelling at roughly 300 miles an hour when it stopped transmitting its position and disappeared, according to data supplied by FlightRadar24. Other aircraft operating out of Tehran were tracked normally, said FlightRadar24 spokesman Ian Petchenik.
The plane came down at 6:18am Wednesday in Sabashahr, near the Iranian capital.
Television footage showed recovery efforts at the crash site, with debris spread across a charred field. Ukraine's foreign ministry said about half those on board held Iranian citizenship and that there were more than 60 Canadian citizens, as well as Swedes, Afghans, Britons and Ukrainians.
Ukraine International Vice President Ihor Sosnovskyi said in Kyiv that the plane, delivered to the airline new in 2016, was in good condition and had its last shop visit on January 6. He said the crew was also very experienced and that there were no indications of human error, while declining to comment on possible reasons for the crash, the company's first since it was set up in 1992.
Under established protocols, Iran would lead any investigation because the crash occurred on its soil. Ukraine Prime Minister Oleksiy Honcharuk said his government has informed Iran that its experts should take part.
Few nations have the expertise and technical facilities to conduct a full accident probe, however, so the US, France, Britain or other countries have provided assistance in the past. The plane's crash-proof recorders will have to be downloaded in a specialist lab.
International rules for air-crash probes mean the countries that made the plane and its engines usually participate, providing technical expertise. But the current turmoil between Iran and the US may complicate the involvement of the NTSB and Boeing.
Hassan Rezaeifar, head of the Civil Aviation Organization'saccident-investigation office, said it is for Iran to decide how and where the plane's so-called black boxes are decoded, "as per the law," according to the semi-official ISNA news agency.
However, Reza Jafarzadeh, a spokesman for the aviation organization, told Bloomberg that his nation observes international protocols, and that Ukraine, "as well as the planemaker," can take part in the probe.
France's BEA hasn't been contacted for assistance but has helped Iran in the past, said a spokesman, Sebastien Barthe.
The tragedy comes at a tense time for both Iran and Boeing.
The Islamic republic fired more than a dozen missiles at US bases in retaliation for the killing of Iran's top general, Qassem Soleimani. Just prior to the crash, US aviation regulators issued new restrictions barring civilian flights over Iraq and Iran.
The US State Department issued a statement offering assistance to Ukraine. "The United States calls for complete cooperation with any investigation into the cause of the crash," according to the statement.
Boeing is still gripped by one of the worst crises in its 103-year history after the two Max disasters led to the grounding of that fleet in March. The company said in a statement that its thoughts are with crew, passengers and their families after the tragedy, without commenting on the cause. The planemaker's stock declined 1.3 per cent to US$332.94 at 11:57am in New York.
The website flightradar24 showed the 737-800 jet left Tehran's Imam Khomeini International Airport for Ukraine's Boryspil International Airport early Wednesday morning local time.
Safran confirmed that the twin-engine plane was powered by turbines from its CFM venture with General Electric, while declining to comment further.