A senior Ukrainian security official said Thursday that his government was looking into four possible causes for the sudden crash of a passenger jet in Iran, including a possible missile strike. Oleksiy Danylov, secretary of Ukraine's national security council, said Ukrainian investigators wanted to search for any missile fragments after information "appeared on the internet" suggesting debris from a Russian anti-aircraft missile was found.

Danylov specifically cited unconfirmed claims online that fragments of a possible Russian-made Tor surface-to-air missile had been discovered.

CBS News transportation correspondent Kris Van Cleave reports that holes in some of the wreckage at the crash site have drawn comparisons to damage done to the Malaysia Air flight that was shot out of the sky over Ukraine more than five years ago. An international team of investigators last year named four key suspects, three Russians and one Ukrainian, who were to be charged with murder over the missile attack that brought Malaysia Airlines Flight 17 down in eastern Ukraine.

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Asked Wednesday whether he could "say categorically" that the Ukrainian jet carrying dozens of Canadians was not shot down, Canadian Prime Minster Justin Trudeau said it was "too early to speculate."

Witnesses say the Ukrainian International Airlines Boeing 737-800 was engulfed in flames before it crashed down just minutes after take-off from Tehran early Wednesday morning. All 176 people on board were killed, most of them Iranian and Canadian nationals.

Нині триває нарада за участю представників компетентних органів Ірану, в тому числі іранського ІСАО - міжнародної... Posted by Олексій Данилов on Thursday, January 9, 2020

The other three possibilities "being studied," according to Danylov, were a mid-air collision with drone or "other flying object," an explosion on the plane "as a result of a terrorist attack," or a technical failure that led to the explosion of an engine.

The plane's crew never made a radio call for help and was trying to turn back to the airport in Tehran when the aircraft dropped off the radar, according to an initial Iranian report released Thursday. The lack of such a distress call suggested a sudden emergency struck the plane. Investigators from Iran's Civil Aviation Organization offered no immediate explanation for the disaster, however.

Eyewitnesses, including the crew of another passing flight, described seeing the plane engulfed in flames before crashing, the report said.

The Reuters news service said a Canadian security source told it there were signs one of the plane's engines had overheated.

The crash caused a massive explosion when the aircraft hit the ground, likely because it had been fully loaded with fuel for the flight to Kyiv, Ukraine's capital. It scattered flaming debris and passengers' belongings across a wide stretch of farmland.

Rescuers work at the crash site of a Boeing 737 Ukrainian passenger plane in southern Tehran, Iran, on Jan. 8, 2020. All 176 passengers and crew on board were confirmed dead. Xinhua News Agency/Ahmad Halabisaz via Getty Images

The report also confirmed that both of the so-called "black boxes" that contain data and cockpit communications from the plane had been recovered, though they were damaged and some parts of their memory was lost. It remained unclear where the boxes would be analyzed, as only a handful of countries have the technical capability to do so.

As Van Cleave reported, the U.S. National Transportation Safety Board would normally assist in such a crash probe, but given the current tension between Tehran and the U.S., that was unlikely to happen.

The crash came soon after Iran launched a ballistic missile attack against Iraqi military bases housing U.S. troops amid a confrontation with Washington over its killing an Iranian Revolutionary Guard general in a drone strike last week.

Ukrainian President Volodymyr Zelenskiy said he planned to call Iranian President Hassan Rouhani about the crash and the investigation.

The plane was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, at least 63 Canadians and 11 Ukrainians, according to officials.

Ukrainian officials, for their part, initially agreed with Iranian suspicions that the 3-1/2-year-old plane was brought down by mechanical trouble but later backed away from that and declined to offer a cause while the investigation is going on.

While the cause of the tragedy remained unknown, the disaster could further damage Boeing's reputation, which has been battered by the furor over two deadly crashes involving a different model of the Boeing jet, the much-newer 737 Max, which has been grounded for nearly 10 months.