Images of a Russian-made surface-to-air missile circulating online have been at the centre of reports that a Ukraine airliner was shot down.
Experts in verification, however, cannot confirm if the images are genuine or exactly when or where they were taken.
Speculation swirled online that an air defence missile might be to blame, but US officials and Canadian Prime Minister Justin Trudeau have said intelligence points to the aircraft having been shot down.
A surface-to-air missile is designed to be launched from the ground to destroy aircraft or other missiles, and is one type of anti-aircraft system.
US officials have said they are now "confident" the Ukraine International Airlines Flight 752 that came down in Tehran was shot down by an Iranian missile , with President Donald Trump saying "somebody could have made a mistake".
The officials said the plane had been tracked by Iranian radar before being shot down by the anti-aircraft missiles.
Data showed the Kiev-bound Boeing was airborne for two minutes after departing Tehran when the heat signatures of two surface-to-air missiles were detected, according to one of the officials.
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Bellingcat researcher Giancarlo Fiorella said it is "too early to say" whether the plane was brought down by a missile.
He added: "It is one of the possibilities it was brought down, but before we make that determination we really have to be careful, take our time and verify the images so we can be sure."
Aviation expert and former head of flight operations for the Civil Aviation Authority (CAA), Mike Vivian, said: "The aircraft was new - relatively - three years old. The crew were well trained. The airline was a good airline with an exemplary safety record - over 20 years.
"The weather was okay, so it is difficult to explain a sudden disappearance, which is what this was."
Oleksiy Danilov, secretary of Ukraine's Security Council, told Ukrainian media officials had four working theories regarding the crash - which killed 176 people including three Britons - including a missile strike.
Ukrainian investigators have arrived in Iran and are awaiting permission to examine the crash site and look for missile fragments.
Mr Danilov said: "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."
Newsweek said a Pentagon official, a senior US intelligence official and an Iraqi intelligence official confirmed Mr Danilov's comments, but the Pentagon has not yet publicly commented.
The Iranian military has disputed suggestions the airliner was brought down by a missile - something Downing Street previously said there was "no indication" of.
However Iran's head of civil aviation was quoted by ISNA News Agency as saying that it was "impossible that a missile hit the Ukrainian plane".
On Thursday, US President Donald Trump said the deadly crash could have been a mistake and he did not believe it was a mechanical issue.
The three US officials said Washington believed the shooting down of the plane was an accident.
An Iranian report said the plane was on fire in the air, citing witnesses on the ground and in a passing aircraft flying at high altitude.