Ukrainian officials were on Thursday considering terrorism, a missile strike and catastrophic engine failure as potential causes for the fatal crash of one of its airliners in Iran, as aviation authorities in Tehran revealed the jetliner was on fire before it came down.
Ukraine's National Security and Defense council chief, Oleksiy Danilov, said a meeting was taking place with Iranian authorities, where various causes behind the crash were 'being studied,' including a theory that the plane was hit by an anti-aircraft missile, according to a statement on Facebook.
Other theories under consideration are whether there were technical problems with one of the plane's engines that caused it to explode, whether the plane could have collided with a drone or 'other flying object,' or whether there was an explosion inside the plane.
The Ukrainian International Airlines (UIA) flight PS752 came down just minutes after takeoff from Tehran on Wednesday, killing all 176 people on board, including dozens of Iranians and Canadians. The Boeing 737-800 was headed for Kiev, where 138 passengers were expected to take a connecting flight to Canada. Ukrainians, Swedes, Afghans, Germans and British nationals were also aboard.
Conflicting claims about potential causes for the disaster began hours after the crash, when Iranian state media blamed technical issues and Ukraine ruled out rocket attacks. Within hours on Wednesday, officials in both countries had walked back those initial statements.
An initial report by the Iranian Civil Aviation Organization on the crash cites witnesses as saying the airliner was on fire while in the air and changed directions after a problem, turning back toward the airport. People on other aircraft at higher altitudes also saw the flames, Iranian officials say. Images of the wreckage show the plane torn to piece, its parts charred and strewn across a field.
The timing of the crash has fueled speculation, coming just hours after Iran fired missiles at two Iraqi bases housing US troops in retaliation for the killing of its top general, also in Iraq. The exchange of attacks between Tehran and Washington on Iraqi soil is a dramatic escalation of tensions between the adversaries, and is raising fears of another proxy war in the Middle East.
Ukraine International Airlines, which had never had a crash before Wednesday, has said it is highly unlikely the cause was error by its crew. UIA was founded in 1992, a year after Ukraine's independence from Moscow.
Tehran's airport is complicated and the pilots required several years of training to use it, said UIA President Yevhenii Dykhne. The captain had 11,600 hours of flying on a Boeing 737 aircraft while the pilot had 12,000 hours on the aircraft.
While it's early to speculate what might have gone wrong, openly available flight data provides some clues.
The Boeing 737-800 climbed to an altitude of nearly 8,000 feet after taking off from Tehran, before the aircraft's data suddenly disappeared, according to flight-tracking service FlightRadar 24. FlightRadar 24 data suggests that the crash happened within three minutes after takeoff.
FlightRadar 24 said said in a tweet the jet had been in service for about three-and-a-half years.
According to former FAA chief of staff Michael Goldfarb, the lost data is 'very unusual' and could suggest a 'catastrophic' incident.
'When the data is lost at 8,000 feet suddenly that implies something catastrophic happened to that airplane and they could no longer communicate with the ground control,' Goldfarb told CNN.
An airliner should be able to keep flying even if one engine fails, which means pilots normally have time to communicate and recover the aircraft.
The Boeing 737-800 is a predecessor to the company's 737 Max, which has been grounded since March after two fatal crashes that killed 346 people.
Goldfarb said an 'uncontained' engine failure -- which would create the type of large fire ball reported by witnesses -- could be to blame, but it was too early to tell. An uncontained engine failure, in which pieces of the engine break apart, releases shrapnel that can destroy the plane.
Alan Diehl, a former accident investigator for the US National Transportation Safety Board, Federal Aviation Administration and Air Force, said that that engine failure would be one of the main focuses of any investigation, considering the aircraft lost contact minutes after takeoff. 'That's the second-most dangerous phase of flight,' he told CNN.
He said investigators always looks at four main causes in a crash -- mechanical, human, environment and lastly hostile actions.
'All of those are certainly on the table,' he said.
Canadian and Swedish authorities have been invited to cooperate in the investigation, the Iranian Civil Aviation Organization said in a statement, while the first joint meeting between Iranian and Ukrainian experts, sent to Tehran to investigate the incident, has already taken place, according to the organization.
But tensions between the United States and Iran are already complicating the probe.
Searchers have found the plane's black boxes -- a cockpit voice recorder and flight data recorder -- which could offer crucial evidence about what happened to the plane leading up to the crash, the Tehran prosecutor told Iranian state media. But Iranian officials don't plan to share information from the black boxes with the plane's manufacturer, US company Boeing, as is usual in crash investigations, Ali Abedzadeh, the head of Iran's Civil Aviation Authority, told the semi-official Mehr news agency.
The US will not be involved at any stage of the investigation, he said.
Under international rules, Iran is responsible for the investigation, but Ukraine should participate in the probe as the state of registry and state of operator. So should the US as the state of design and manufacture of the Boeing aircraft.
According to Goldfarb, Iran refusing to work with Boeing would make it impossible to carry out a fair and thorough investigation.
'They have to work with Boeing. Boeing has all the data, owns all the drawings and designs, they have the engineers, they know the plane,' he said.