A plane that crashed in Iran with 176 people on board was trying to return to the airport, Iranian investigators say.
The Ukraine International Airlines Boeing 737-800 went down just minutes after taking off from Tehran's airport on Wednesday, leaving no survivors.
An initial probe found the aircraft experienced a problem as it was leaving the airport zone, and was "on fire".
Ukraine said it was examining various possible causes, including an anti-aircraft missile strike.
On Thursday, Oleksiy Danylov, the secretary of Ukraine's security and defence council, said in a Facebook post (in Ukrainian) that three other main hypotheses were being considered:
- a mid-air collision with a drone or other flying object
- engine destruction/explosion due to technical reasons
- an explosion inside the plane as a result of a terror attack
Mr Danylov said Ukrainian investigators, who are in Iran, wanted to search for possible debris from a missile at the site of the crash. Iran is known to have Russian missile defence systems.
The investigation would include experts who worked on the investigation into the 2014 downing of Malaysia Airlines flight MH17 in eastern Ukraine, Mr Danylov added.
The crash came just hours after Iran carried out missile strikes on two airbases housing US forces in Iraq.
However, there is no evidence the two events are linked.
Amid tensions heightened by the US killing of top Iranian general Qasem Soleimani on 3 January, Iran has said it will not hand over the recovered black box flight recorders to Boeing or to the US.
Under global aviation rules Iran has the right to lead the investigation, but manufacturers are typically involved.
'No distress calls'
Iran's Civil Aviation Organisation (CAOI) chief Ali Abedzadeh said: "The plane, which was initially headed west to leave the airport zone, turned right following a problem and was headed back to the airport at the moment of the crash."
Mr Abedzadeh added that witnesses saw the plane "on fire" before the crash, and that pilots hadn't made any distress calls before trying to return to Imam Khomeini airport.
"Several domestic and foreign flights were flying in Iranian space at the same altitude of 8,000ft (2,400m). The issue of the missile's impact on the aircraft cannot be true in any way," he said.
Mr Abedzadeh said the initial findings had been sent to Ukraine and the US, where Boeing has its headquarters.
He was earlier quoted as saying that "terrorism" had played no role in the crash.
Sweden and Canada had also been sent the findings, as their nationals were on board, he added.
Who is investigating the crash?
Normally, the US National Transportation Safety Board would have a role to play in any international investigations involving US-made Boeings. But the board must act with permission and in accordance with legislation of the foreign country concerned.
In comments published by Iran's Mehr news agency, Mr Abedzadeh, said: "We will not give the black box to the manufacturer and the Americans.
"This accident will be investigated by Iran's aviation organisation but the Ukrainians can also be present," he added.
Mr Abedzadeh said it was not yet clear which country would analyse the black boxes - a cockpit voice recorder and flight data recorder.
Ukrainian President Volodymyr Zelensky earlier said that "a thorough and independent investigation will be conducted in accordance with international law", and that he would speak to Iranian leaders to step up co-operation in investigating the crash.
Ukraine is observing a day of national mourning on Thursday.
Boeing said it was "ready to assist in any way needed", while Canadian Prime Minister Justin Trudeau said his country expected to have a role in the investigation and had offered technical assistance.
The Ukraine International Airlines flight PS752 to Kyiv had 167 passengers and nine Ukrainian crew on board.
The majority of passengers were from Iran and Canada.
Ukraine's Tehran embassy initially blamed engine failure but later removed the statement, saying any comment regarding the cause of the accident prior to a commission's inquiry was not official.
There was good visibility when the plane went down near Iran's capital, according to the Flightradar24 aviation website. Officials from the airline said the crew were experienced.
Who was on board?
Among the victims were 82 Iranians, 63 Canadians, 11 Ukrainians including all nine crew, 10 Swedes, four Afghans, three Britons and three Germans, Ukrainian Foreign Minister Vadym Prystaiko said. Fifteen of the dead were children.
But the German government later said "we currently have no knowledge that German citizens are among the victims of the plane crash in Iran".
Iran's head of emergency operations said 147 of the victims were Iranian. That would suggest that 65 of the foreign nationals had dual nationalities. The Ukrainian airline gave a helpline number for further information about passengers: +38-044-581-50-19.
Mr Trudeau said 138 passengers on the flight were en route to Canada via Kyiv.
"All people who won't be coming home to their parents, their friends, their colleagues or their family," he said. "All had so much potential, so much life ahead of them".
What went wrong?
Flight data from the Ukrainian Airlines Boeing 737-800 is openly available online. It shows that the plane climbed normally after taking-off from Tehran.
It reached nearly 8,000ft before the aircraft's data suddenly disappears.
This is unusual and would suggest some type of catastrophic incident on board the plane. We have no evidence at this stage to tell us what caused the incident.
According to a former air crash investigator, any suggestion of engine failure feels premature. This possibility can't be ruled out at this early stage but an airliner such as the Boeing 737-800 is designed to keep flying if there is an engine failure.
Plus, if there was a failure then we would normally expect the flight data to show the plane's climb becoming less steep.
Have you been personally affected by this story? Please get in touch with us by emailing firstname.lastname@example.org.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: