The crash occurred hours after Iran fired a missile strike at US forces in Iraq, in retaliation for the killing of a top Iranian general.

According to US officials and others, the early evidence suggest the flight, a Boeing 737-800 was shot down by Iranian missiles.

We have intelligence from multiple sources including our allies and our own intelligence.

The evidence indicates that the plane was shot down by an Iranian surface to air missile.

This may well have been unintentional.

This new information reinforces the need for a thorough investigation into this matter.

The process for determining the cause of a plane crash can take months or even years, and often involves coordination between different countries.

But this time with heightened tensions between the US and Iran, the process is facing complications.

Iranian officials had earlier said the plane suffered a technical fault.

A spokeswoman for the Ukrainian president Volodymyr Zelensky said, "Ukraine is interested in finding the truth."

"Therefore, I ask all international partners: If you have any evidence to assist the investigation, please provide it."

International protocol lays down specific guidelines for conducting post-crash investigations.

It's a very fluid process.

It's concurrent actions.

A lot of things are taking place at the same time.

Dr. Daniel Kwasi Adjekum is a former investigator with more than two decades of experience in aviation.

Investigations are usually conducted under the auspices of the government of the country where the crash takes place.

When the country of occurrence is taking the lead of the accident, the country of registry and operator of the aircraft will be notified so that they can send in what we call an accredited representative to be part of the investigation.

The country of design and manufacture of the aircraft is going to part of it, okay?

So these are the major stakeholders.

In a more typical accident probe involving a US-manufactured plane at least some US safety experts would be on their way to the site within a day or two.

But given the diplomatic and security issues clouding this accident, decisions about US participation are in limbo.

Dr. Adjekum said that countries with a high portion of citizens aboard the flight may also become involved in the investigation.

Even under less fraught circumstances, collaboration between multiple countries can be difficult.

Dr. Adjekum said that in previous crashes like Ethiopia Airlines flight 302, healthy diplomatic relations made it safe for the two countries to collaborate.

But even then, investigators tussled over where to send the plane's black boxes.

But in this case because of the particular situation where the two of them cannot reconcile, a third party entity might suffice.

So what could happen is that in such a situation ICAO might broker a third party.

Most likely the French or the Germans might be brought in by the Iranians to do the read out.

Once an investigation shifts into national security or military issues, civil investigators generally are relegated to a marginal role.

If they have hard evidences based on maybe satellite photography which sort of irrefutable, then the dynamics change, it becomes a criminal investigation.

When that happens, international treaties about data sharing and broad participation no longer apply.

And participants tend to be focused on protecting intelligence sources.

Despite the hurdles, Iranian authorities have indicated through intermediaries, including the Civil Aviation arm of the United Nations, they want the US National Transportation Safety Board to provide technical assistance.

In the normal course, this also would mean some level of participation from the Federal Aviation Administration, Boeing and General Electric company, which made the plane's jets with a French joint venture partner.