To civil aviation professionals, including pilots, engineers and former crash investigators, there was something immediately puzzling about the crash of the Ukraine International Airlines passenger jet that fell burning out of the sky minutes after takeoff from Tehran.

Conversation on forums, and in a risk assessment that was rapidly produced by the organisations OpsGroup, pointed to the sudden and catastrophic nature of the event, including the loss of both communications and tracking systems.

Fail-safe systems that would have allowed the aircraft to get back safely in the event of engine failure appeared to have been compromised in an instant. Others pointed to what looked like penetrating holes in the airframe, leading some to compare them to the damage suffered by the Malaysian Airlines flight MH17 shot down over Ukraine by a Buk surface-to-air missile five years ago.

By Thursday morning, there was another piece in the puzzle to contend with: an unverified photograph from the crash site purporting to show the “seeker head” from a Russian-built Tor-M1 surface-to-air missile of the kind used by Iranian air defence units.

Hours later those suspicions appeared to become more concrete as US defence and intelligence officials supplied the alleged missing link that many had been waiting for.

They disclosed that the US had picked up the signature of an anti-aircraft missile battery locking on to the Ukrainian plane, and then the infrared heat signal of two missile launches followed by that of an explosion on the plane.

By Thursday evening that assessment was supported by Canada, where many of the victims came from. The prime minister, Justin Trudeau, said: “We have intelligence from multiple sources – including our allies and our own intelligence: the evidence indicates that the plane was shot down by an Iranian surface-to-air missile.”

Shortly after Boris Johnson said there was now a “body of information” that indicated the Tehran plane crash was caused by an Iranian missile.

The statements come amid tensions between the US and Iran following the American assassination of the head of the al-Quds Force in Baghdad and the Iranian retaliation in a missile barrage targeting US forces in Iraq in the hours ahead of the crash.

Aviation insiders had already been warning against a rival theory, that the relatively new Boeing 737-800, which had been serviced just two days before, could have suffered a so-called uncontained engine failure, where debris is blown out of a failing engine into the fuselage and damages key systems.

In three of these incidents – two involving Boeing jets owned by Southwest Airlines, and one involving a Qantas Airbus taking off from Singapore in 2010 – the aircraft landed safety with only one death, thanks to the significant “redundancy”, or fail-safe designs, built into modern planes to allow them to land safely after an engine failure.

Among those troubled by details of the crash was Larry Vance, a former aviation investigator with the Transportation Safety Board of Canada, who told Canada’s Globe and Mail newspaper that “simple engine failure” could not explain the crash.

“That just doesn’t fit the scenario at all,” he said. “Here we had some kind of event that knocked the transponder off the plane. Some kind of event that disabled the electronics to that system. It takes a lot to disable the electronics on a sophisticated aircraft like the 737-800.”

OpsGroup, an aviation safety information sharing site set up immediately after MH17 was downed in 2014, warned operators that until further contrary evidence emerged the Ukrainian crash should be treated as a potential “shootdown”.

The reality, as was pointed out by aviation forums, was that it had happened before, not just in 2014 with MH17 but also in 1988 when the USS Vincennes, an American guided-missile destroyer, shot down an Iranian airliner, killing everyone on board.

The unverified picture of the seeker head of the Tor-M1 missile seemed to some to be too good to be true, lying on the ground and largely intact, but accounts circulating on social media also included claims that bangs had been heard by residents on the ground before the crash.

The images of the crash debris were also pored over. While some apparent evidence of fragment damage to the aircraft turned out to be debris from the ground, other images showed ragged holes in one of the engines and scorching to one side of the cockpit, and other parts of the aircraft.

That seemed inconsistent with a massive accidental fuel leak and fire of the kind that brought down the Air France Concorde at Charles de Gaulle airport in 2000 after a fuel tank was punctured by debris on the runway.

While US officials would not disclose the intelligence they claim to have that indicates an Iranian missile was to blame, they acknowledged the existence of satellites and other sensors in the region, as well as the likelihood of communications intercepts and other similar intelligence.

It is an assessment, however, that starkly contradicts Iran’s own preliminary investigative report, which was released Thursday. That claimed the aircraft was trying to turn back for the airport when it went down, a claim greeted with some scepticism due to the lack of communications with the jet making it impossible to judge whether the air crew were actively piloting the plane or whether it simply swung off course as it plunged towards the ground.