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By Nancy A. Youssef in Washington, Aresu Eqbaliin Tehran and Rory Jones in Dubai 

The U.S. believes that a Ukrainian commercial aircraft that crashed shortly after takeoff from Tehran on Wednesday was downed by two surface-to-air missiles fired by Iran, a U.S. official said.

"We have a high level of confidence that this was shot down by Iran," the U.S. official said, adding that the plane was being tracked by Iranian radar just before the two missiles were fired.

A second U.S. official said the U.S. believes Iran may have shot down the aircraft by mistake.

One factor contributing to the U.S. assessment that the plane had been shot down was the large field of debris at the crash site, the second official said. Planes that crash as a result of mechanical failure have narrower debris, that official said.

President Trump on Thursday said "I have my suspicions" about the crash, voicing doubt in remarks at the White House that the cause may have been mechanical problems.

"I personally don't think that's even a question, personally," he said. "It was flying in a pretty rough neighborhood and somebody could have made a mistake."

The Ukraine International Airlines jetliner crashed shortly after takeoff from Tehran on Wednesday, killing all 176 passengers and crew members on board. The crash came amid military clashes between the U.S. and Iran.

The Boeing Co. 737-800 single-aisle jet crashed after departing the Iranian capital's Imam Khomeini International Airport en route to Kyiv, Ukraine.

The investigation into what downed the flight is expected to be particularly thorny, with heightened tensions between the U.S. and Iran over the killing of a top Iranian general likely to complicate international cooperation.

Iranian officials, quoted by the state news agency, said the plane had suffered a technical fault, that it had started to turn back in the direction of Tehran airport and was already on fire before it hit the ground. They said the jet had reached 8,000 feet before disappearing from radar.

Ukrainian officials said they were considering a number of possible scenarios, although cautioned they were only preliminary theories at this stage. Those include a strike by an antiaircraft missile, a collision with a drone, an engine explosion or a blast inside the aircraft as a result of a terrorist attack.

No evidence was given for a potential attack. A spokesman for Ukraine International Airlines declined to comment on "hypotheses."

The crash came hours after Iran launched missile attacks on U.S. troops at two Iraqi bases in retaliation for the killing of Iranian Maj. Gen. Qassem Soleimani. The timing of those attacks raised questions about whether the Ukrainian airliner was downed by a projectile.

A U.S. official familiar with the matter said Wednesday that data transmitted via satellite indicated everything was normal on the jetliner until the sudden loss of data and the fatal dive. That data suggest to some U.S. air-safety officials that there may have been some sort of hostile act, said the person, who also cautioned that it was too early to draw conclusions.

Iran rejected the notion of a hostile act. "Since the Ukrainian pilot was trying to return to the airport, a rocket, missile strike or the country's defense system is out of question," said Hassan Rezaifar, head of Iran's Civil Aviation Organization commission for accidents investigation, quoted by Iran's state news agency. "No missile part was found in the crash scene."

The last high-profile shootdown of a commercial airliner occurred in eastern Ukraine in 2014, when pro-Russian separatists battling the central government downed a Malaysian plane with a surface-to-air missile. All 298 people on board the Boeing 777 died.

A team of 45 Ukrainian experts and officials arrived in Tehran early Thursday and will be involved in decoding the plane's black boxes and identifying and repatriating bodies. The team had already collected DNA samples from relatives of the victims in Ukraine.

"The priority for Ukraine is to establish the causes of the crash," Ukrainian President Volodymyr Zelensky said. Ukraine said Mr. Zelensky spoke by phone with Iranian President Hassan Rouhani, who pledged full cooperation in the investigation, including sharing all data with Ukrainian officials on the ground in Tehran.

However, it isn't clear how much access U.S. investigators will get to the crash site. Iran's Mr. Rezaifar said the probe would be conducted to international standards set by the International Civil Aviation Organization.

That would indicate U.S. companies and entities could be involved. However, Ali Abedzadeh, head of Iran's Civil Aviation Organization, told the semiofficial Mehr News Agency on Wednesday that his organization wouldn't provide Boeing or the U.S. access to the black box.

Based on the International Civil Aviation Organization's convention, the home country of the airline and the makers of the plane and its major systems are each entitled to appoint a representative to participate in crash probes.

Boeing has said it is in contact with Ukraine International Airlines and is ready to assist in any way. Determining what went wrong is critical to the plane maker, which already is dealing with the grounding of its best-selling 737 MAX fleet after two fatal crashes involving that jet.

The plane that crashed, a 737-800, is the most popular version of the aerospace giant's workhorse jet. The model and its variants account for around 25% of all commercial jetliners in operation, enjoying one of the industry's best safety records.

Georgi Kantchev in Moscow contributed to this article.

Write to Rory Jones at rory.jones@wsj.com and Georgi Kantchev at georgi.kantchev@wsj.com

(END) Dow Jones Newswires

January 09, 2020 13:03 ET (18:03 GMT)

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