New details are emerging about Wednesday's plane crash in Iran as investigators hunt for clues as to what brought down a Ukrainian-operated Boeing 737 minutes after takeoff in Tehran, killing all 176 people on board.
A Ukrainian official said Thursday that multiple potential causes for the crash of Ukraine International Airlines flight PS752 were being investigated, including terrorism, a missile strike, or catastrophic engine failure.
The development came as an initial report by the Iranian Civil Aviation said the jetliner was on fire before it crashed. The Iranian report, citing witnesses, said the plane also changed directions after a problem and turned back toward the airport.
Ukraine's National Security and Defence council chief, Oleksiy Danilov, wrote on Facebook that a meeting was taking place Thursday 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.
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 an technical problems with the plane's engine that caused it to explode, or whether there was an explosion inside the plane.
Conflicting claims about potential causes for the disaster began hours after the crash, when Iranian state media blamed technical issues for the crash and Ukraine ruled out rocket attacks. Within hours on Wednesday, officials in both countries had walked back those initial statements.
Ukraine International Airlines (UIA) defended its record on Wednesday, saying that given the level of the flight crew's experience, it was highly unlikely the crash was the result of error. This is the carrier's first crash. 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.
The crash came hours after Iran fired missiles at two Iraqi bases housing US troops in retaliation for the killing of its general, leading to speculation over the timing of the incident.
A fireball in the sky
The Boeing 737 took off early Wednesday. It was headed to the Ukrainian capital of Kiev with 167 passengers and nine crew members.
Witnesses have described seeing a fireball in the sky before the plane crashed. Images of the wreckage show charred parts of the plane strewn over a field.
The victims are from several nations, including 82 Iranians, 63 Canadians, 11 Ukrainians, 10 Swedes, four Afghans, three Germans and three British nationals, Ukrainian officials said.
The loss has led to emotional scenes in at Boryspil International Airport in Kiev on Thursday, where mourners lay flowers at a makeshift memorial in the arrivals hall.
Ukrainian President Volodymyr Zelensky has declared Thursday a day of national mourning.
Iran refuses to work with US
As authorities work to determine the cause of the crash, tensions between the United States and Iran are threatening to complicate the investigation.
Hostilities between Tehran and Washington have escalated after Iran fired missiles at Iraqi bases housing American troops. The missiles were in retaliation for a US strike that killed Iran's top commander, Qasem Soleimani, on Iraqi soil last week.
While it's unclear whether the incidents are related to the plane crash, tensions between the two adversaries are spilling over to the investigation.
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.
"We will not give the black box to the manufacturer or America," 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.
There's no way Iran could refuse to work with Boeing and do justice to the investigation, Former FAA chief of staff Michael Goldfarb said.
"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.
Plane is a predecessor to the 737 Max
The Boeing 737-800 jet had been in service for about three and a half years, according to FlightRadar 24. It suggests the crash happened within minutes after takeoff, and the plane had climbed to an altitude of nearly 8,000 feet before the aircraft's data suddenly disappeared.
That is "very unusual" and suggests a "catastrophic" incident, as opposed to engine failure, Goldfarb said.
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.