Iran has refused to cooperate with the US over the investigation into the Boeing jet that crashed on Wednesday, killing all 176 people on board within minutes of take-off from Tehran’s Imam Khomeini airport.
The fully-fuelled Ukraine International Airlines flight PS752 came down in a field and caught fire in an incident that the Iranian authorities immediately attributed to a mechanical fault.
Rescuers were unable to reach the wreckage strewn across a field close to the town of Khalaj Abad because of the intensity of the fire and there were no survivors.
The crash came just hours after Iran launched missile attacks on US targets in Iraq and several airlines announced that they were avoiding airspace above the two countries due to safety concerns.
Speculation that Iranian air defences could have brought down the three-year-old Boeing 737-800, bound for Kiev, heightened after Ukraine withdrew a statement that confirmed that mechanical fault was to blame and the country’s president called for a criminal investigation.
Tensions between Iran and the US led to an immediate setback in the investigation after Iran said that it would not work with Boeing and would not hand over flight data from the two recovered black box flight recorders.
"This accident will be investigated by Iran's aviation organisation but the Ukrainians can also be present during the incident's investigation," said Ali Abedzadeh, the head of Iran’s civil aviation authority.
Under international aviation rules, air crash investigations are led by the country where the accident happened. But few nations have the expertise to conduct a full accident probe so the US, France or the UK often help.
US Secretary of State Mike Pompeo called for "complete cooperation with any investigation into the cause of the crash".
Ukraine announced it was sending a team to investigate.
"Our priority is to establish the truth and those responsible for this terrible catastrophe," Ukrainian President Volodymyr Zelenskiy said in a statement on Facebook.
Mike Vivian, the former head of the UK’s Civil Aviation Authority, said he found it difficult to believe that engine trouble was responsible because modern jets are capable of gliding short distances without any power.
“It’s suddenly come out of the sky and more significantly without any radio warning of distress,” he told UK broadcaster Sky News.
“Often when you get a mid-air breakup you see the debris on the radar screen but that doesn’t appear to be the case here.
“They’ll be looking for evidence of mechanical failure but I find it difficult, troubling, to suggest that engine failure may have caused this,” he added.
“They will also be looking for evidence of foul play – whether there was possibly an internal device or whether there was a missile or something fired from the ground at the aircraft and the wreckage should reveal that.
“It’s a puzzling accident this because the normal culprits like weather, like maintenance, like old aircraft or possibly crew training – I don’t believe they’re present in this case.”
Ukraine said passengers and crew included 82 Iranians, 63 Canadians and 11 Ukrainians. Ten Swedes, four Afghans, three Germans and three Britons are also believed to have died.
“My sincere condolences to the relatives and friends of all passengers and crew,” President Zelenskiy said.
He ordered the prosecutor general to launch a criminal investigation and said Ukraine will test the airworthiness of its entire fleet of civilian aircraft, “no matter the conclusions about the crash in Iran”.
At Kiev International Airport, where the flight was due to arrive, the father of Ihor Matkov, a crew member, said he had found about a crash in a phone call from his daughter.
“She asked where Ihor was flying to, and we said ‘Tehran’ and that he would return on the morning flight,” said Valery Matkov. “And then we found the video of the plane crashing.”
The crash and death of 63 Canadians is one of the country’s worst aviation disasters. Prime Minister Justin Trudeau said Canada would "work closely with its international partners to ensure that this crash is thoroughly investigated, and that Canadians' questions are answered".
The Swedish victims included two families with children. Swedish Foreign Minister Ann Linde said: “My warmest condolences to the relatives. It is a tragedy for the victims, their families and friends.”
At a briefing in Kiev, Ukrainian Prime Minister Oleksiy Honcharuk cautioned against speculation that the aircraft had been deliberately brought down pending the results of the investigation.
Ukraine International Airlines said the aircraft, built in 2016, was serviced just two days before the accident.
The crash is the first fatal accident for Ukraine International Airlines, founded in 1992 after the collapse of the Soviet Union.
Flight data showed the Boeing 737-800 aircraft taking off for Kiev on Wednesday morning before it stopped sending data almost immediately afterwards.
The Boeing 737-800 is a ubiquitous twin-engine jetliner used for short to medium-range flights. Thousands of the planes are used by airlines around the world and it has a good safety record.
Modern aircraft are designed to cope with an engine failure shortly after take-off and to fly for extended periods on one engine.
CFM, which manufactured the plane’s engines, said any speculation regarding the cause of the crash was premature.
"We have no further information at this time. Any speculation regarding the cause is premature," the company said in a statement.