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By Aresu Eqbali in Tehran and Rory Jones in Dubai 

Iran said the Ukrainian airliner that crashed outside of an airport in Tehran, killing all 176 people on board, was shot down unintentionally by its armed forces.

Iranian officials had initially denied claims by Western officials that the Boeing Co. 737-800 jet was downed by a missile system fired by Iran, possibly by mistake.

The single-aisle jet crashed Wednesday four hours after Iran had launched military strikes against the U.S. in Iraq in retaliation for the killing of a top general, and the Middle Eastern state was on high alert for a potential U.S. response.

The Ukraine International Airlines plane approached a sensitive military base operated by the Islamic Revolutionary Guard Corps and was downed due to "human error," Iran's general staff of the armed forces said in a statement on state television early Saturday local time.

The armed forces detected greater U.S. air activity and U.S. warplanes on its radar after the Iranian strikes on American forces in Iraq, the statement said.

Iranian President Hassan Rouhani tweeted that the downing was a "great tragedy & unforgivable mistake."

Ukrainian President Volodymyr Zelensky said Kyiv expects Tehran to punish those responsible and pay compensation.

"This wasn't a good morning, but it brought the truth," he said in a Facebook post. "We expect from Iran assurances of a full and open investigation, bringing those responsible to justice, the return of the bodies of the dead, payment of compensation [and] official apologies through diplomatic channels."

The airline didn't immediately respond to a request for comment.

Iran's prosecutor general, Mohammad-Jafar Montazeri, in a directive to the military prosecutor, demanded "swift, meticulous" investigations to identify all the causes and individuals involved in the incident, according to the semiofficial ISNA news agency.

Iranian Foreign Minister Javad Zarif said the error was caused by "U.S. adventurism" in the Middle East. "Our profound regrets, apologies and condolences to our people, to the families of all victims, and to other affected nations," he tweeted.

Since the Iranian strikes, President Trump has said the U.S. didn't plan to respond to Iran and that Iranian forces appeared to be standing down from further conflict with American forces.

The downing of the jet, which killed a number of Iranians, is likely to place further pressure on Iran's leadership, already challenged by U.S. sanctions that have prompted an economic crisis and widespread public protest.

Until Saturday, Iranian officials had repeatedly denied that the jet was shot down by its armed forces, despite claims by U.S., U.K. and Canadian officials that it crashed after being hit by missiles.

Iranian investigators had said the plane likely crashed due to a technical error and the pilot was attempting to return to Tehran's main international airport when it hit the ground.

On Saturday, Iranian investigators said they would send the so-called black-box data to France for analysis after determining that Iran doesn't have the necessary technology to assess the data.

Hassan Rezaifar, the head of the accident investigation bureau at Iran's Civil Aviation Organization, said Iranian investigators asked Canada, France and the U.S. to send equipment to Iran, but the countries didn't accept the proposal.

Instead, Iran asked five countries--Ukraine, Sweden, Britain, Canada and the U.S.--whether the data could be downloaded in a credible and neutral laboratory and all five agreed to analyze the black boxes in France, Mr. Rezaifar said, according to state news agency IRNA. He added that these decisions were made before the statement from Iran's armed forces.

"Now that the reason of the incident has been identified, the decision should be made if it is needed to download the black box information or not," he told the state news agency. "The point is that the black box will not be opened in Iran."

Tehran gave Ukrainian investigators access to the black-box data and cockpit voice recorders on Friday, though Ukrainian officials said they have yet to start analyzing them. A team of 45 Ukrainian experts is on the ground in Tehran, Ukrainian officials said.

Iranian officials also have invited investigators from Boeing, the U.S., Ukraine, France and Canada to probe the causes of Wednesday's crash, which killed a number of Canadians.

The National Transportation Safety Board, the U.S. entity tasked with transport-accident investigations, said it is monitoring the situation and evaluating its level of participation.

Boeing has started preparing the necessary paperwork and has been in touch with both the State and Commerce departments, The Wall Street Journal reported Thursday.

In the normal course of an investigation, some level of participation would also be expected from the Federal Aviation Administration and General Electric Co., which made the plane's engines with a French joint-venture partner.

Early Saturday, Canadian Prime Minister Justin Trudeau said he expects "full cooperation" from Iranian authorities to ensure Canadian officials take part in an investigation into why the plane was shot down. The country said it lost 57 citizens in the crash, down from an earlier estimate of 63.

"Our focus remains closure, accountability, transparency, and justice for the families and loved ones of the victims," Mr. Trudeau said in a statement.

U.S. industry and government officials on Friday said the Iranians have the ability to conduct routine downloads of black-box recorders, but are skeptical they have the technical expertise and experience to extract data from damaged devices.

The crash comes after Iran has undergone a convulsive few months. At least 59 people died this week in a stampede at a funeral march for Maj. Gen. Qassem Soleimani, the military commander killed by a U.S. strike.

In November, hundreds of demonstrators were killed by security forces when they rose up across the country in protest against government austerity measures, prompted partly by American sanctions. The security response was the deadliest in decades.

The Ukrainian jet isn't the first time tensions between the U.S. and Iran have led to an aviation tragedy. In 1988, the U.S. Navy's Vincennes warship downed Iran Air Flight 655 with an air-defense missile.

U.S. officials said they mistook the Airbus SE A300 airliner for a warplane they feared would attack the ship. All 290 people, mostly Iranian, on the plane died, prompting an outpouring of national grief in Iran.

Paul Vieira in Ottawa contributed to this article.

Write to Rory Jones at rory.jones@wsj.com

(END) Dow Jones Newswires

January 11, 2020 02:51 ET (07:51 GMT)

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