WASHINGTON — When a Ukrainian passenger jet crashed outside Tehran, Iran, shortly after taking off on Wednesday morning, speculation immediately turned to the conflict between Iran and the United States. And while details remain scant, U.S. authorities believe that it was “highly likely” that an errant Iranian missile brought down the aircraft.

Ukrainian authorities are also coming to that view, after initially seeming to accept the Iranian view that engine failure was at work.

While determining the cause of a crash can take months, if not years, Iranian authorities in the hours since the accident appear to have added to initial suspicions by refusing to allow the kind of collaborative investigation that is commonplace when a civilian aircraft suffers a serious accident.

That has not kept others from reaching conclusions. CBS News and Newsweek both reported on Thursday that Pentagon and intelligence officials now believe that the Iranians, fearful of an American air assault, mistakenly shot down the Ukrainian jet, which was full of civilians.

President Trump endorsed this view, telling reporters Thursday morning that Iran “could have made a mistake.” He discounted “mechanical” issues as potentially at work.

Ukrainians appear to increasingly share his skepticism of Iranian explanations. Oleksiy Danilov, an official on Ukraine’s National Security and Defense Council, said on Thursday that the Ukrainian government will look at reports that a Russian-made missile may have taken down the plane.

The aircraft, a Boeing 737-800 narrow-body jet operated by Ukraine International Airlines, was en route to Kyiv, the Ukrainian capital. Flight PS752 — as it was known according to international aviation codes — climbed to an altitude of 7,900 feet but then inexplicably plummeted to the ground east of Tehran. Photos on social media, witness video and reports from Iranian news sites showed the fuselage heavily charred by fire.

All 176 people onboard the airplane perished.

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Rescue workers carry the body of a victim killed in the plane crash near Tehran, Iran, on Wednesday. (Photo: Ebrahim Noroozi/AP)

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Iranian authorities have suggested that engine failure was a factor, though no evidence has been presented to substantiate that claim. The Ukrainian Embassy in Iran issued — and then deleted — a statement attributing the crash to engine problems.

An article on the semiofficial Fars news agency claimed the “Boeing 737 passenger planes are notorious for frequent technical issues.”

While Boeing has had problems with its 737 Max aircraft, the 737-800 line is among the most reliable in the world. Its safety record, says veteran aviation industry analyst Robert W. Mann, is “excellent.”

Suspicions about what happened to Flight PS752 are especially high because the crash comes amid a military standoff between Iran and the United States, precipitated by the killing of Iranian Gen. Qassem Soleimani by an American drone in Iraq last week.

Shortly before the airplane went down, Iran retaliated for the killing of Soleimani by firing its Fateh-110 and Qiam-1 ballistic missiles at two military bases in Iraq where U.S. troops are stationed.

An increasingly popular hypothesis among Western observers is that Iran potentially shot down the civilian aircraft, perhaps mistaking it for an American fighter plane. While there is no direct evidence yet that an Iranian missile took down PS752, aviation experts have struggled to come up with any other plausible reason for the accident.

OpsGroup, a consultancy focused on aviation safety, published a blog post that included an analysis of photos that seemed to show the fuselage of PS752 punctured in many places. Internal damage to the craft would not have caused such puncturing.

“We would recommend the starting assumption to be that this was a shootdown event,” the post read.

Debris from the Ukrainian airliner that crashed Wednesday in Shahedshahr, Iran. (Photo: Mahmoud Hosseini/picture alliance via Getty Images)

Iran has long worked to bolster its air defenses, in part by purchasing Russian systems called S-300PMU-2, and also by designing defense systems of its own. Iran recently deployed the homegrown Bavar-373 long-range battery, whose Sayyad-4 missile can travel about 186 miles.

An accidental shootdown of a civilian aircraft would be unusual but not unprecedented.

On July 3, 1988, the passengers aboard Iran Air Flight 655 were expecting a short and easy morning flight to Dubai, only to tragically fall victim to geopolitics.

Stationed in the Strait of Hormuz, the U.S. Navy cruiser USS Vincennes mistakenly identified the Iran Air jet as an attacking aircraft. The two SM-2MR missiles fired by the Vincennes at 10:54 a.m. killed all 290 onboard, including 66 children.

Then, as now, the assumption hostilities between the United States and Iran were high, and a U.S. helicopter had taken incoming fire from an Iranian boat that same morning. But the assumption of an attack was also disastrously wrong.

More recently, Russian-backed separatists in Ukraine were accused of shooting down Malaysia Airlines Flight 17 in 2014, killing all 298 people onboard. The investigation was hindered by pro-Russian forces and the Kremlin itself.

The crash of Flight PS752 may be unrelated to the hostilities between the United States and Iran. But aviation experts believe that Iran’s anxiety about an American attack on Tehran could account for what happened to the Ukrainian jet.

At the very least, suggestions of engine malfunction made by Iranians have been met by vicious skepticism.

Jeff Wise, an aviation expert who authored “The Taking of MH370,” about the Malaysia Airlines passenger jet that went missing over the Indian Ocean in 2014, told Yahoo News that suggestions of engine failure were “horses***,” and not a believable cause for the accident.

He notes that the Boeing 737 flown by Ukraine International Airlines was relatively new and that pilot Vladimir Gaponenko was relatively experienced, having logged 11,600 flying hours. Also, the airplane had been serviced only two days before.

A rescue worker searches the scene where a Ukrainian plane crashed southwest of Tehran, Iran. (Photo: Ebrahim Noroozi/AP)

Even if one engine failed, the other would have continued to power the plane, and the crew would have had time to issue a distress signal. Also, catastrophic engine failure likely wouldn’t turn the entire jet into a fireball, which is what PS52 appeared to become in its final moments.

Writing for the New York magazine website, Wise speculated that “Iranian air-defense forces would have been on high alert,” especially since the capital city of Tehran “would be an obvious target. Memories remain fresh of U.S. airstrikes against Baghdad at the start of its wars against Iraq.”

That could have led Iran’s military to mistake Flight PS752 for a hostile aircraft, much as the USS Vincennes had done with the Iranian plane in 1988.

“A couple of hours in, that’s the horse at the front of the race,” Wise told Yahoo News, though he also cautioned that “there could well be a cause here we’re missing.”

If the crash were an accident, the cause would likely be more complex than just an engine failure. Aviation accidents often progress according to the Swiss cheese model pioneered by James Reason, in which a number of small, highly improbable errors just happen to coincide, compounding and eventually causing a catastrophe.

Some have pointed to a 2018 accident involving Southwest Airlines Flight 1380, a Boeing 737-700, over eastern Pennsylvania. In that incident, an engine failed and started to come apart, damaging the airplane and killing one person, who was partially pulled out of a cabin window. But even with one engine gone, pilot Tammie Jo Shults safely landed the plane in Philadelphia.

A similar incident is not likely to have brought to PS752 says Patrick Smith, a former airline pilot who runs the popular Ask the Pilot blog on civil aviation.

People stand near the wreckage after a Ukrainian plane carrying 176 passengers and crew members crashed near Imam Khomeini International Airport on Jan. 8. (Photo: Rouhollah Vahdati/ISNA/AFP via Getty Images)

“It’s doubtful that even a rapid or explosive decompression would result in a crash, unless somehow it resulted in very serious structural damage,” Smith told Yahoo News. Nor did he see any evidence for the kind of fuel tank explosion that brought down TWA Flight 800 (a Boeing 747-100 with 230 passengers onboard) over Long Island Sound in 1996.

Much like Wise, Smith considered an inadvertent shootdown as a plausible explanation, writing that circumstances of the crash were “suspicious.”

On Twitter, Breaking Aviation News shared a photo of what it said was a “missile head” at the crash site. The photo had allegedly been obtained from Iranian sources, but there was no way to independently verify that report.

Iranian authorities, for their part, have said they will not share the airplane’s flight data and cockpit voice recorder, which they recovered after the crash, with Boeing, which as the manufacturer of the aircraft would customarily be part of the investigation. Iran’s civil aviation chief, Ali Abedzadeh, has suggested that Ukraine would be allowed to partake in the investigation, though he did not specify how or to what extent.

Canada also wants a share of the investigative duties, since 63 Canadians were killed in the crash. “We are going to make sure that we are a substantive contributor to this investigation,” Canadian Prime Minister Justin Trudeau said on Wednesday.

Representatives for Boeing would not discuss particulars of the accident. In response to a query, the company sent Yahoo News an email expressing sympathy to the victims’ families and a readiness to work with Ukraine International Airlines on the investigation.

The prospects of a collaboration between Iran and Western partners is virtually nonexistent. “Normally, one would wait for independent investigators to do their jobs,” says Mann, the aviation industry analyst. “That may not be a realistic expectation in this case.”

A spokesperson with the Federal Aviation Administration who asked to remain anonymous because they were not authorized to speak about the issue, said that, according to international rules, “the U.S. government would have to be invited by the Iranian government to participate.”

Passengers’ belongings are seen at the site where the Ukraine International Airlines plane crashed after takeoff from Iran’s Imam Khomeini airport, on the outskirts of Tehran, Iran. (Photo: Nazanin Tabatabaee/WANA (West Asia News Agency) via Reuters)

In the meantime, aviation experts reached their own conclusions, absent a full investigation.

Mann added that, in Iran, “reports of a rapid pace of accident scene cleanup will further reduce the ability to objectively determine accident cause. Without access to crash scene or data recorders, analysis becomes speculative and hypothetical, a very opaque situation that comes at an unfortunate time geopolitically, and for Boeing.”

Boeing has faced tough scrutiny from Congress and the American public for its handling of the 737 Max crisis, which resulted in two fatal crashes, one of a plane flown by Lion Air, an Indonesian company, and the other involving Ethiopian Airlines. Since then, reports have revealed that Boeing knew about problems with the Max navigation system, known as MCAS, but proceeded with production anyway.

The Ukraine International Airlines version of the Boeing aircraft, however, did not have a MCAS system.

The fallout from that crisis led to the firing of Boeing chief executive Dennis Muilenburg in late December.

Iran is not a country known for openness, and that means speculation over what happened will only intensify. “If the lack of transparency persists,” says Mann, we may never find out whether the accident cause was mechanical failure, a terrorist incident” or an intentional shootdown.

Perhaps inevitably, the cause of the crash even became a matter of political contention on Capitol Hill, with House Minority Leader Kevin McCarthy, R-Calif., urging House Democrats to delay a vote on curtailing Trump’s ability to wage war with Iran until Iranian authorities released the voice and data recorders from Flight PS752.

“I think the Democrats should pause,” McCarthy said, until more is known about Iran’s potential involvement in the crash.

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