Iran announced early Saturday that it had accidentally shot down a Ukrainian passenger jet, blaming human error because of what it called the plane’s sharp, unexpected turn toward a sensitive military base, according to a statement issued by the country’s military.

The announcement reversed Iran’s claims that mechanical issues caused the crash of the aircraft on Wednesday, which killed all 176 people aboard. It had persistently denied that Iranian military defenses had downed the aircraft, a Boeing 737-800.

The Iranian military’s statement said the plane “took the flying posture and altitude of an enemy target” as it came close to an Islamic Revolutionary Guards Corps base. It said that “under these circumstances, because of human error,” the plane “came under fire.”

The statement said that the person responsible for shooting down the plane would face legal consequences, and that the military would undertake “major reform in operations of all armed forces” to make sure that such an error never happened again. It said Revolutionary Guards officials had been ordered to appear on state media and give the public a full explanation.

On Twitter, President Hassan Rouhani of Iran said “investigations continue to identify & prosecute this great tragedy & unforgivable mistake.”

In a statement of his own, Iran’s foreign minister, Mohammad Javad Zarif, tried to place some of the blame on the United States, saying on Twitter that the disaster was “caused by U.S. adventurism led to disaster.”

International pressure had been building on Iran to take responsibility. American and allied intelligence assessments have said that Iranian missiles brought down the plane, most likely by accident, amid the heightened tensions between the United States and Iran.

State television in Iran aired footage that it said showed two black boxes recovered from the crash site. Processing their data could take more than a month, and the investigation could take up to two years, Hassan Rezaeifar, the head of the Iranian investigation team, said Friday.

The State Department had no immediate comment late Friday about Iran’s admission of responsibility.

On social media, Iranians began expressing anger toward the military soon after the announcement, many of them using the term “harshest revenge,” which officials had repeatedly promised in the wake of the American drone strike that killed Maj. Gen. Qassim Suleimani, a powerful Revolutionary Guards commander, last week.

“They were supposed to take their harsh revenge against America, not the people,” wrote Mojtaba Fathi, a journalist.

Suspicions that an Iranian missile had brought down the plane began immediately after the crash Wednesday morning — just hours after Iran fired missiles at two bases in Iraq housing American forces.

The Iranians had asked the National Transportation Safety Board to help with the investigation, and the State Department had granted waivers to allow the American agency to help. A senior administration official said Friday that he thought the Iranians wanted American investigators there to keep up the appearance that they did not know what had caused the crash.

The official, who spoke on the condition of anonymity because he was not authorized to discuss these matters publicly, said the Iranian military had poor command and control, and this was reflected in what had happened with the airplane. Communications among officials and between units are often lacking, he said, and confusion can be the norm. Western analysts often overestimate the capability of parts of the Iranian military, he said.

Official Iranian news media had cited technical problems as the cause in the hours after the crash. Iran’s Civil Aviation Organization chief, Ali Abedzadeh, doubled down on that assessment on Friday, saying nothing could be determined until the data from the black boxes was analyzed, and he characterized statements made by other nations as politically motivated.

The military announcement came as something of a surprise. As late as Friday night, officials were weighing whether to blame faulty jet equipment in acknowledging that Iranian missiles brought down the jet, according to four Iranians familiar with the deliberations.

Until Saturday, Ukraine’s main intelligence agency, known as the S.B.U., said only that it had narrowed the cause of the crash to a missile strike or a terrorist act and that it could not confirm Western intelligence that an Iranian missile system was likely to blame.

An Iranian report released on Thursday said that the plane, bound for the Ukrainian capital, Kyiv, was in flames before it hit the ground but sent no distress signal.

VideoThe New York Times has obtained and verified video showing the moment a Ukrainian airliner was hit in Iran.CreditCredit…Screenshot from video

Video verified by The New York Times and published on Thursday appeared to show a missile fired from Iranian territory hitting the plane, Ukraine International Airlines Flight 752. The video showed a small explosion as the plane flew above Parand, a city near the airport — where it stopped transmitting its signal before it crashed. The plane turned back toward the airport before it exploded and crashed, other videos verified by The Times showed.

When Iran began firing missiles early on Wednesday in retaliation for the killing of Maj. Gen. Qassim Suleimani by the United States in Baghdad, international airlines rerouted flights away from Iran, and the Federal Aviation Administration barred American carriers from the airspace in the region.

After the crash, experts raised questions about why Iranian authorities had not stopped flights in and out of Tehran.

In Iran, a debate over how much blame the government bears threatened to destroy the national solidarity that followed the country’s conflict with the United States. Many Iranians said that their anger over the lack of accountability at the highest levels of government had quickly returned.

On Friday, Secretary of State Mike Pompeo said that the United States and its allies had intelligence showing that the passenger jet had been shot down.

“We do believe that it’s likely that the plane was shot down by an Iranian missile,” Mr. Pompeo said at a briefing at the White House announcing new sanctions against Iran. “We’re going to let the investigation play out before we make a final determination. It’s important that we get to the bottom of it.”

Mr. Pompeo was the first American official to publicly confirm the intelligence assessments.

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 57 Canadians among its 176 passengers and crew.

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

President Volodymyr Zelensky of Ukraine made clear on Friday that Western governments had not initially shared the evidence underpinning their assessments that Iran had brought down the Ukrainian jet, though later a spokeswoman said that American officials had handed over more information.

Ukrainian officials also analyzed the plane’s flight pattern on Friday and determined it had stayed within the normal corridor for flights out of Tehran’s Imam Khomeini International Airport, Ukraine’s foreign minister, Vadym Prystaiko, said at a news conference.

The crash has presented Mr. Zelensky, a 41-year-old comedian who swept into office with a surprising election victory last spring, with the most urgent crisis of his short tenure. And its aftermath has the potential to open a fresh rift between Ukraine and its most important Western allies.

Mr. Zelensky has already turned into an unwilling player in United States domestic politics as a result of President Trump’s pressure campaign seeking announcements of investigations by Ukraine that could benefit him politically. Now, Mr. Zelensky is stuck in the middle of an even more volatile American crisis: the conflict with Iran.

Mr. Zelensky needs Iranian cooperation to deliver the full-fledged investigation of the disaster that he has pledged to Ukrainians. But he also needs the data collected by Western intelligence — as well as continued Western support in Ukraine’s conflict with Russia.

“Our goal is to ascertain the undeniable truth,” Mr. Zelensky said in a statement on Friday. “We believe this is the responsibility of the whole international community before the families of the dead and the memory of the victims of the catastrophe.”

Reporting was contributed by Niraj Chokshi, Anton Troianovski, Julian E. Barnes, Karen Zraick, Edward Wong and Michael Levenson.

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