WASHINGTON — American and allied officials said Thursday that they had intelligence that missiles fired by Iranian military forces were responsible for the downing of a Ukrainian jetliner in Iran and the deaths of all aboard this week in Iran, most likely by accident.

The disclosures suggested that the deaths were a consequence of the heightened tensions between Washington and Tehran that have played out since a U.S. drone strike killed a top Iranian general last week.

Prime Minister Justin Trudeau of Canada, citing a preliminary review of the evidence, called for a full investigation “to be convinced beyond all doubt.” The jetliner was carrying 63 Canadians among its some 176 passengers and crew.

“We recognize that this may have been done accidentally,” Trudeau said at a news conference in Ottawa, Ontario. “The evidence suggests very clearly a possible and probable cause for the crash.”

President Donald Trump, speaking earlier at the White House, said only that he suspected that the downing of the plane was the result of “a mistake on the other side.” Senior U.S. officials were more forthcoming, saying they had a high level of confidence in their findings. U.S. intelligence agencies determined that a Russian-made Iranian air defense system fired two surface-to-air missiles at the plane, one official said.

And video verified by The New York Times appeared to show an Iranian missile exploding near a plane above Parand, near Tehran’s airport, the area where the jetliner, Ukrainian Airlines Flight 752, stopped transmitting its signal before it crashed.

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The passengers aboard the plane likely faced horrifying final moments, starting with the explosion as the missiles detonated in the air just outside the plane, sending shrapnel and debris spiraling through the fuselage. The plane turned back toward the airport, then began its uncontrolled descent toward the ground.

American satellites, designed to track missile launches, detected the firing of the Iranian short-range interceptor. U.S. intelligence agencies later picked up Iranian communications confirming that the system brought down the Ukrainian airliner, officials said.

An initial Iranian report released Thursday said that the plane, bound for the Ukrainian capital of Kyiv, was in flames before it hit the ground but sent no distress signal. A security camera captured its impact: first the predawn darkness, then a series of blinding bursts of light in the distance, followed by a storm of burning debris in the foreground.

Even before world leaders and U.S. officials confirmed the intelligence assessment, the mysterious circumstances of the disaster had raised suspicions that a missile brought down the airliner. The crash occurred hours after Iran launched a barrage of ballistic missiles at U.S. military targets in Iraq, and Tehran, bracing for possible American retaliation, readied its ample air defense system.

After Iran began firing missiles early Wednesday in retaliation for the killing of Gen. Qassem Soleimani, international airlines rerouted flights away from Iran, and the Federal Aviation Administration barred American carriers from the airspace in the region. The new information about the tragic mistake with its air defense systems raised questions about why Iranian authorities had not stopped flights in and out of Tehran.

Iran denied that its military was responsible for the crash of the plane, a Boeing 737. Ali Rabiei, an Iranian government spokesman, called it “a big lie” and blamed the accusations on “psychological warfare” against Tehran.

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“The United States is making the pain of the families worse,” Rabiei said in a statement.

Iranian officials questioned the Western account, saying the plane would have exploded if hit by a missile. The air defense system used Wednesday, however, is designed to explode near aircraft, creating shrapnel that takes a plane out of the sky, rather than directly hitting it.

In addition to denying responsibility, Iran invited the National Transportation Safety Board of the United States to assist in the investigation despite previous reports that the Americans would not be involved, according to correspondence reviewed by The Times. The board assigned an investigator to the crash, a spokesman said Thursday evening.

Iranian authorities recovered the plane’s “black box” flight data recorders, but they were damaged by the crash and fire, the Iranian report said. That raised the possibility that some of the information stored in them electronically had been destroyed, but investigators can retrieve useful data even from damaged recorders.

Iran also invited Boeing, the jet’s manufacturer, to help investigate the black box, a government spokesman said, according to Iran’s official news agency, IRNA. Boeing is “supporting the NTSB in the investigation,” said a spokesman, Gordon Johndroe.

Sanctions against Iran prevent Boeing from contacting its government without an export license, and Johndroe said the company is applying for one.

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Canadian investigators were also arranging to visit the crash site, a senior government official said.

Ukraine was negotiating with Iran to allow investigators to search the site for possible rocket fragments, Oleksiy Danilov, secretary of the National Security and Defense Council of Ukraine, told Censor.net, a Ukrainian news outlet.

Ukrainian officials want “to find out the causes of the tragedy,” President Volodymyr Zelenskiy of Ukraine said in a videotaped address released Thursday. “We will definitely find out the truth. We will conduct a detailed and independent investigation.”

Zelenskiy also sought to preempt criticism of Ukrainian authorities for allowing the flight to take off soon after the Iranian attacks on American targets in Iraq. He said that Tehran’s airport had been operating as usual at the time, noting that other European airlines were taking off and landing.

Evidence gathered by American and allied intelligence contradicted Iran’s denials.

The U.S. military’s Space-Based Infrared System, which relies on satellites in various orbits to track the launch and flight path of ballistic missiles, detected the missile launch. While American missile defense sensors are primarily meant to defend against long-range launches, they can often detect launches of air defense systems, including those designed to work at low altitudes, officials have said.

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On Wednesday, U.S. officials combined the information from the satellites with intelligence from intercepted calls to determine what brought down the plane.

The infrared system had also detected the anti-aircraft missile fired by Russian-supported separatist fighters in eastern Ukraine in 2014 that brought down Malaysia Airlines Flight 17, officials said at the time. All 298 people aboard were killed.

Russia first sold Iran the air defense system — which NATO calls SA-15 and Russia refers to as Tor — in 2005, prompting American protest. Designed to operate at medium to low altitudes and intercept both aircraft and guided weapons, the Iranian military could have positioned the defense system to defend the airport if officials believed the U.S. military was intending to counterattack after Iran’s ballistic missile strikes.

Three to four people operate the system, tracking nearby aircraft by radar. But determining friendly civilian aircraft takes skill, and mistakes are possible, particularly in charged situations.

Echoing Trudeau, Prime Minister Boris Johnson of Britain confirmed that intelligence pinned the shoot-down on the Iranian government and that it “may well have been unintentional.”

Trump was more evasive earlier in the day. “Somebody could have made a mistake on the other side,” Trump said. “It was flying in a pretty rough neighborhood and somebody could have made a mistake.”

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His reluctance to assign blame may be an attempt to avoid inflaming tensions at a time when both governments were taking steps to de-escalate the military confrontation of recent days. The revelations about the intelligence prompted accusations that the U.S. military’s killing of Soleimani set off a chain of events that led to Iran accidentally downing the jet.

“This is the responsibility of the Iranians,” said Sen. Jack Reed of Rhode Island, the senior Democrat on the Armed Services Committee. “But the context was a situation in which they were preparing themselves for a possible attack by the United States. This might not have happened two or three weeks ago.”

The crash in Iran also came at a difficult time for Boeing. The 737 MAX, a new version of the 737 that was downed in Iran, has been grounded for 10 months after two deadly crashes caused in part by new software on the plane.

The crisis is consuming Boeing, which ousted its chief executive late last year and is temporarily shutting down the factory that makes the Max this month. Had investigators found that mechanical problems caused the crash in Iran, it could have raised new questions about the safety of the company’s aircraft. On Wednesday, before the cause was determined, Boeing’s interim chief executive, Greg Smith, sent an email to employees expressing condolences for the victims and pledging to cooperate with investigators.

“The safety of our people, products and services, and all those who fly on what we build, is of the greatest importance to all of us at Boeing,” he wrote, according to a copy of the email reviewed by The Times. “Thank you for your ongoing focus on our values, including safety, quality and integrity.”

At Boryspil International Airport near Kyiv, where Flight 752 had been due to land, grieving flight attendants tended to candles set on the floor in front of a makeshift memorial to the nine crew members who died.

A flight attendant named Tatyana, who declined to give her last name because she was not authorized to speak to the news media, said she visited the memorial Wednesday evening to pay her respects.

“Of course there were concerns, risks to those flights,” she said. “We took this responsibility upon ourselves when we joined the airline — to be ready for anything to happen.”