Iran has invoked an international agreement to get assistance from other countries — including the U.S. — in investigating Wednesday’s fiery crash of a jetliner near Tehran, two people familiar with the matter said.

Such a move is common in aviation disasters, and gives the country where the plane was made the ability to participate. The crashed jet was a Boeing 737-800. A Ukrainian investigation team arrived in Tehran on Thursday, according to Iran’s Ministry of Roads and Urban Development. Other countries have also been invited, it said on its website without elaborating.

American agencies including the National Transportation Safety Board are weighing whether it is legal to engage with Iranian authorities under the terms of sanctions the U.S. has imposed on Iran. Moreover, they are concerned about the safety of sending people there given the confrontation between the two countries that has led to military strikes on both sides, the people said.

The investigation into Ukraine International Airlines Flight 752, which plunged from the sky minutes after takeoff, killing all 176 people aboard, is fraught with difficulty and intrigue.

Conflicting signals have emerged about what might have caused the disaster, with Iranian authorities initially blaming technical issues and then an engine fire. The Ukrainian embassy in Tehran ruled out terrorism, and then amended its statement to offer no comment on possible causes. Ukraine International said it didn’t “even consider” the possibility of crew error.

The crash came hours after an Iranian missile attack on two Iraqi military bases in retaliation for an American drone attack last week that killed one of Iran’s top generals. American forces are stationed at both bases.

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Under the United Nation’s International Civil Aviation Organization, crash investigations are conducted by the nation in which they occur. In addition, the country where the plane and key components are manufactured are allowed to take part.

There were 82 Iranians, 63 Canadians, 11 Ukrainians as well as Swedes, Afghans, Britons and Germans on board the plane, which was delivered new to Ukraine International in 2016.

Iran notified the U.N. agency of the accident in the hours after it occurred, according to the people, who were briefed on the matter but weren’t authorized to speak about it and asked not to be named. The recently serviced, three-year-old jet went down without a distress call and after its global-positioning transmissions were cut off midair, which is unusual for a crash.

A video purportedly shot by a bystander shows flames coming from the jet as it streaked across the night sky and burst into a fireball on impact. “The plane is on fire,” an unidentified male can be heard saying. “In the name of God, God help, call the firefighters.”

The NTSB routinely participates in dozens of crash investigations around the world under the ICAO process, known as Annex 13.

By notifying the ICAO, Iran suggested it might be open to U.S. help in the probe, said the two people. But the Islamic Republic has sent mixed signals, with some officials being quoted as saying they would not allow Americans to analyze the plane’s two crash-proof flight recorders, for example.

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American law also prohibits the NTSB from working in Iran because of longstanding bans on conducting business in that country. The NTSB has occasionally assisted in accident investigations there, but had to obtain special permission from the U.S. Treasury. The process of obtaining such approval has at times taken more than a year.

“The NTSB is monitoring developments surrounding the crash of Ukraine International flight 752 and is following its standard procedures for international aviation accident investigations, including long-standing restrictions under the country embargoes,” the agency said in an emailed statement Wednesday.

“As part of its usual procedures, the NTSB is working with the State Department and other agencies to determine the best course of action.”

The State Department issued a statement offering assistance to Ukraine, but notably didn’t mention helping Iran. “The United States calls for complete cooperation with any investigation into the cause of the crash,” the department said.

Despite early reports from Iran suggesting an engine fire might have led to the crash, some aviation safety experts said the plane’s sudden fall as it apparently was engulfed in fire might have been from a bomb or missile. One veteran aviation accident investigator said the flight-tracking data and amateur video were unlike typical engine failure or fire scenario.

“Airplanes don’t just catch fire and have that fire spread like that in such a short period of time, unless there was an intentional act causing that fire and explosion,” Jeffrey Guzzetti, former chief of the U.S. Federal Aviation Administration’s accident investigation division said in an interview.

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The aircraft climbed normally until it reached an altitude of 7,900 feet (2,408 meters), then suddenly stopped transmitting its position, according to data from the tracking site FlightRadar24.

The tragedy evoked memories of the 2014 downing of a Malaysia Airlines jet by a surface-to-air missile during the conflict in Crimea, eventually blamed by investigators on pro-Russia rebels. Ghasem Biniaz, head of communications for Iran’s Ministry of Roads and Urban Development, said reports of a missile attack are false, state-run IRNA reported.

The Ukraine International jetliner was equipped with a device that communicated with the airline and it showed the plane was behaving normally until it stopped transmitting at about the same time, said a person familiar with the data.

If the amateur video is genuine, it indicates that the plane was essentially a fireball as it plunged, Guzzetti said. Bright lights emanating from the aircraft suggest explosions as well.

Two other aviation safety experts were cautious about jumping to conclusions.

“I wouldn’t take anything off the table right now,” said John Cox, a former airline pilot who is president of Safety Operating Systems. When asked about the potential for a bomb or missile, Cox said, “Certainly they are going to look at external forces.”

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“Accident investigations should stand above politics,” Cox said. “I’m hopeful that all the governments will put politics aside and let a transparent accident investigation take place.”

Ukraine International said in a statement that the investigation would include representatives of Iran, Boeing, the airline and the National Bureau of Air Accidents Investigation of Ukraine.

It’s at least theoretically possible for a fast-moving cargo fire to take down a plane, said Roger Cox, a former NTSB investigator. A Valujet plane crashed in Florida in 1996 after pure oxygen caused a raging inferno, he said. But in that case, the plane flew for about 10 minutes, far longer than the Ukrainian jet.

The tragedy strikes during a deeply challenging period for Boeing, which is gripped by one of the worst crises in its 103-year history. Deadly crashes involving the company’s 737 MAX in Indonesia and Ethiopia led to the global grounding of the jet in March, and the manufacturer is still struggling to get it back into service.

Boeing’s shares slide 1.8% on Wednesday, their biggest loss since Dec. 16. The company hasn’t commented on possible reasons for the crash.

©2020 Bloomberg L.P.