Amid heightened tensions between the United States and Iran, one question loomed over the crash of a Boeing passenger jet in Iran on Wednesday: Who will investigate?

Everyone on board the Kyiv-bound Ukraine International Airlines flight was killed in the crash, which occurred shortly after takeoff from Tehran’s Imam Khomeini airport, including many Iranians and Canadians. Iranian officials blamed the crash on technical difficulties. The Ukrainian Embassy in Iran initially published a statement that ruled out terrorism, but it was later taken down.

Ukrainian President Volodymyr Zelensky urged the public “to refrain from speculating and making uncorroborated theories.”

Past crashes involving U.S. plane maker Boeing’s aircraft have compelled U.S. foes to set aside their resentments and allow American experts access to crash sites or to crucial evidence, because those countries lacked the capabilities or experience to conduct complex investigations alone.

But amid the spike in hostilities in the Middle East, any such understanding between Iran and the United States appeared far from evident Wednesday. Only hours before the crash, Iran had launched ballistic missiles at bases housing U.S. military personnel in Iraq, in retaliation for the U.S. killing of Quds Force commander Qasem Soleimani last week.

Iran said Wednesday that it would not hand over the flight recorders, or “black boxes,” to Boeing, according to the semiofficial Mehr News Agency. The quick rejection of assistance probably also signaled unwillingness to cooperate with other U.S. authorities, such as the National Transportation Safety Board (NTSB).

It is also unclear whether the NTSB, other U.S. agencies or Boeing would be willing or able to take part in an Iranian investigation, even if requested by Tehran. (The United States is diplomatically represented by the Swiss Embassy in Tehran.)

In a statement, the NTSB said it was “monitoring developments” and “following its standard procedures for international aviation accident investigations, including long-standing restrictions under the country embargoes.”

“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 statement read.

The head of Iran’s Civil Aviation Organization, Ali Abedzadeh, said it was unclear which country would analyze the Boeing 737-800′s black boxes, according to Mehr. Ukrainian Foreign Minister Vadym Prystaiko said he and his Iranian counterpart have agreed “to coordinate further actions of our investigation groups closely to determine the cause of the terrible plane crash,” according to a tweet cited by Reuters.

But it’s unclear whether Ukraine possesses all the capabilities needed to aid the investigation. Germany, for instance, declined Ethiopia’s request to analyze the black boxes of a crashed Ethiopian Airlines plane last year because German authorities believed that officials in the African country lacked the necessary preparation and skills.

European lawmakers were among the first to call for an international investigation Wednesday. “This has to be examined internationally,” Omid Nouripour, a foreign affairs spokesman for Germany’s Green Party, told broadcaster NTV.

The engines of the crashed Ukraine International Airlines plane reportedly were produced by a company partially based in France.

After the Ethiopian Airlines crash, for instance, Ethiopia requested multiple European nations to analyze the plane’s black boxes. French investigators agreed to do so in the end, as they have particularly extensive experience with some of the most modern Airbus planes that are manufactured in France.

France’s analysis of the Ethiopian Airlines black boxes appeared significant at the time because the plane was a Boeing 737 Max 8, not an Airbus jet.

The French investigative authority for civil aviation safety rejected claims that it was seeking to create a competitive advantage for Boeing competitor Airbus by assigning blame to the U.S. plane manufacturer. The results of such analyses — including when U.S. authorities provide assistance to foreign states — are usually secret and can be released only by the country officially in charge of the investigation.

At times, political disputes have pushed countries where incidents occurred to hand over responsibility of investigations to third parties.

However, the norm, according to Annex 13 of the Convention on International Civil Aviation, is that states where an accident occurs are in charge of the investigations, even though the NTSB may “designate a U.S. Accredited Representative and appoint advisors to carry out the Obligations, receive the Entitlements, provide Consultation, and receive Safety Recommendations from the state of occurrence.” Iran ratified the convention in 1950.

Other parties to that convention have in the past opted to set aside differences with the United States and allow private or official U.S. representatives to be involved.

For example, the crash of a 39-year-old Boeing 737 after takeoff from Havana last year occurred amid deteriorating relations between the Trump administration and Cuba. Still, Cuban officials notified the NTSB, which appointed a representative.

Only one passenger survived the crash, in which 112 people died. The Cuban-led commission — supported by U.S. and Mexican officials — later contradicted prior conclusions and argued that errors by the plane’s crew “in the calculations of weight and balance” were to blame for the crash.

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