The fiery crash of a Ukrainian airliner shortly after take-off from Tehran, killing all 176 people aboard, was likely caused by a technical malfunction, intelligence sources say.
The Ukraine International Airlines Boeing 737-800, en route to Kiev and carrying mostly Iranians and Iranian-Canadians, crashed on Wednesday, hours after Iran fired missiles at bases housing US forces in Iraq.
The timing has lead to speculation the plane may have been hit.
But five security sources – three Americans, one European and one Canadian – who have asked not to be named, have told Reuters the initial assessment of Western intelligence agencies was that the plane had suffered a technical malfunction and had not been brought down by a missile.
There was evidence one of the jet's engines had overheated, the Canadian source said.
The crash comes at a difficult time for planemaker Boeing Co, which has grounded its 737 MAX fleet after two crashes.
The 737-800 is one of the world's most-flown models with a good safety record and does not have the software feature implicated in crashes of the 737 MAX.
"We are in contact with our airline customers and stand by them in this difficult time," the manufacturer said in a statement earlier on Wednesday.
"We are ready to assist in any way needed."
In Paris on Wednesday morning, the maker of the plane's engines, French-US firm CFM said speculation regarding the cause was premature.
Smouldering parts and debris, including shoes and clothes, were strewn across a field southwest of the Iranian capital, where rescue workers in face masks laid out scores of body bags.
Among the victims were 82 Iranians, 63 Canadians and 11 Ukrainians, Ukrainian authorities said.
The Tehran-Toronto via Kiev route was a popular one for Canadians of Iranian descent visiting Iran, in the absence of direct flights, and carried many students and academics heading home from the holidays.
The victims included a newlywed couple that had gone to Iran to get married.
Arash Pourzarabi, 26, and Pouneh Gourji, 25, were graduate students in computer science at the University of Alberta. Four members of their wedding party were also on board.
At Kiev's main airport, candles and flowers were laid next to pictures of deceased Ukrainian crew members.
It was Kiev-based Ukraine International Airlines' first fatal crash and the carrier said it was doing everything possible to establish the cause.
Ukraine said it was sending a team of experts to Iran to investigate.
President Volodymyr Zelenskiy said he had instructed Ukraine's prosecutor general to open criminal proceedings, without specifying who they would involve.
Canadian Prime Minister Justin Trudeau said Canada would "continue to work closely with its international partners to ensure the crash is thoroughly investigated" and US Secretary of State Mike Pompeo said the United States was calling for complete co-operation with any investigation into cause.
Under international rules, responsibility for investigating the crash lies with Iran. Iranian state television said both of the plane's black box voice and data recorders had been found.