The attack was planned by the group, who armed themselves with weapons and used a level of violence that can only suggest they intended to kill him
Iran retaliated for the January 3 US drone strike in Baghdad that killed a top Iranian general.
The Iranian government said the missile strike scenario made "no sense," however, arguing that several internal and international flights had been sharing approximately the same airspace.
"This government will not rest until we get that."
Canada's transportation safety board on Thursday said it had accepted an invitation from Iran's civil aviation authority to join the inquiry.
Britain's Johnson called Thursday for a full, transparent investigation.
Trump would not directly confirm what US intelligence was saying privately.
"I have my suspicions," Trump said, adding that "somebody could have made a mistake."
But unnamed officials told US media that satellite, radar and electronic data indicated Tehran's air defense units downed the aircraft.
ABC News reported that an unnamed official said it was "highly likely" the plane was brought down by two SAMs.
Ukraine called for United Nations support for a broad investigation, and sent 45 crash investigators to Tehran to take part in the inquiry led by Iranian authorities.
Investigators are pursuing several possibilities, including engine failure, a missile strike or an act of terror.
"If any country has information that can help conduct a transparent and objective investigation into the tragedy, we are ready to receive it and cooperate in further verification," the Ukraine presidency said in an English-language statement.
Ali Abedzadeh, head of Iran's civil aviation organization and deputy transport minister, said Iran and Ukraine were "downloading information" from the aircraft's black boxes retrieved from the crash site.
"But if more specialized work is required to extract and analyze the data, we can do it in France or another country," he said.
Analysts were examining photographs posted online of the wreckage and a private video apparently taken of the flight when it was struck for evidence that it was downed by a missile.
"I think this has a very good possibility of being accurate," John Goglia, a former US aviation safety expert on the National Transportation Safety Board, said of the missile theory.
"Airplanes that have just taken off and have made a climb to 8,000 feet, that's entering the safest period of time in the flight. So even an engine failure at that altitude should not cause the type of event we've just observed," he told AFP.
The Ukrainian airline crash brought back memories of another tragedy, involving a US military error.
In 1988, an Iran Air flight was mistakenly shot down over the Gulf by a surface-to-air missile fired from the US warship USS Vincennes.
All 290 people aboard, most of them Iranians, were killed.
The attack was planned by the group, who armed themselves with weapons and used a level of violence that can only suggest they intended to kill him
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