The crew of the Ukrainian plane which crashed near Tehran did not make a radio call for help and were trying to turn back when it came down, investigators in Iran have said.
Iran's civil aviation authority made the comments in a preliminary report - a day after all 176 people on board the Ukraine International Airlines flight were killed, including three British nationals .
Investigators also said that the plane was engulfed in fire before it crashed.
The crash caused a massive explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel for the flight to the Ukrainian capital Kiev, the report said.
It also confirmed that both of the black boxes that contain data and cockpit communications had been recovered, though they had been damaged and some parts of their memory was lost.
Oleksiy Danilov, secretary of Ukraine's Security Council, told Ukrainian media that officials had several working theories regarding the crash, including a missile strike.
Ukrainian investigators have arrived in Iran and are awaiting permission to examine the crash site and look for missile fragments.
"A strike by a missile, possibly a Tor missile system, is among the main (theories), as information has surfaced on the internet about elements of a missile being found near the site of the crash," Mr Danilov said.
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The Iranian military has disputed suggestions the airliner was brought down by a missile - something Downing Street also says there is "no indication" of.
Mr Danilov also said other possible causes under consideration included a drone or another flying object crashing into the plane, a terrorist attack or an engine malfunction causing an explosion.
Canada's prime minister has vowed his government will get answers after 63 Canadians were among those who died.
Justin Trudeau revealed 138 passengers on board the Boeing 737-800 plane were connecting to Canada.
The plane crashed minutes after taking off from the Iranian capital Tehran and hours after Iran launched ballistic missile attacks on two Iraqi bases housing US soldiers.
Officials in Iran have blamed an engine fire, while the Ukrainian president Volodymyr Zelenskiy has ordered a criminal investigation into the crash.
Mr Zelenskiy has also said he will speak to his Iranian counterpart by phone to step up co-operation to establish the truth about the plane crash.
The Ukrainian president has declared 9 January an official day of mourning in Ukraine, while the presidential office said several possible causes of the crash are being discussed.
Mr Zelenskiy has urged people to refrain from "manipulation, speculation, conspiracy and hasty evaluations" in relation to the crash.
The plane was on fire immediately prior to the incident, according to an initial report by Iran's civil aviation organisation.
Witnesses on the ground and in a passing aircraft said the jet was on fire while still aloft.
"It's dangerous to speculate on possible causes," Mr Trudeau said on Wednesday.
He added that the Canadian victims' families want and deserve answers about what happened.
"Know that all Canadians are grieving with you," Mr Trudeau said.
The flight included many international students, a family of four and newlyweds.
Nine members of crew and at least 26 children were also on board, according to the flight manifest.
Mr Trudeau said his foreign minister was in touch with the Ukrainian government and his transport minister is reaching out to his international counterparts.
Getting answers from Iran might prove difficult as Canada closed its embassy in Iran in 2012 and suspended diplomatic relations.
The British victims of the crash included engineer Sam Zokaei, from Surrey, Saeed Tahmasebi Khademasadi, from west London, and Mohammad Reza Kadkhoda Zadeh, from near Brighton.
Mr Tahmasebi, 35, worked for engineering firm Laing O'Rourke, who described him as a "popular and well-respected engineer" who will be "missed by many of his colleagues".
Mr Tahmasebi's wife, 34-year-old Iranian Niloufar Ebrahim, was also named among the casualties of the crash.
Mr Zokaei, 42, from Twickenham, worked for BP as a senior reservoir engineer, with the company saying: "We are shocked and deeply saddened by this tragic loss of our friend and colleague and all of our thoughts are with his family and friends."
Mohamma Reza Kadkhoda-Zadeh, from near Brighton, was a father of one who was on his way back to the UK after visiting his family over the Christmas period in his native Iran.
The 40-year-old lived in Brighton and ran a dry cleaning business called Mr Smarty's in nearby Hassocks.
Ukraine's foreign minister Vadym Prystaiko said the flight also included 82 Iranians, 11 Ukrainians, 10 people from Sweden, four passengers from Afghanistan and three from Germany.
The crash comes amid escalating tension in the Persian Gulf following the killing last week of Iranian General Qassem Soleimani by the US, prompting several airlines to reroute flights away from Tehran's airspace.
The Foreign Office has advised against all travel to Iran for British-Iranian nationals, with a slightly downgraded warning advising against all but essential travel for British nationals.