KYIV, Ukraine — Investigators will look into the possibility that a missile shot down the Ukrainian passenger jet that crashed in Iran, a senior Ukrainian official said on Thursday, but he did not rule out a range of other possibilities for the disaster that killed at least 176 people.
The official, Oleksiy Danilov, secretary of the National Security and Defense Council of Ukraine, said that investigators were following up on unconfirmed reports that fragments of a Russian-made Tor surface-to-air missile — a system used by Iran — had been found near where the plane came down.
Ukraine was negotiating with Iran to allow the investigators to search the crash site near Tehran for possible rocket fragments, he told Censor.net, a Ukrainian news outlet.
The possibilities of a terrorist act, a collision with an airborne object such as a drone, and an engine explosion were also being examined as potential causes of the crash, Mr. Danilov said on his Facebook page.
Ukraine brings unique experience to bear on the case: In 2014, after Russian-backed separatists took control of parts of eastern Ukraine, an antiaircraft missile that international investigators later said was Russian-made, shot down Malaysia Airlines Flight 17 there, killing all 298 people aboard.
The Ukrainian airliner that went down on Wednesday morning had turned back toward the Tehran airport before it crashed in a huge explosion minutes after takeoff, according to an initial Iranian report released on Thursday. It said that the plane, a Boeing 737-800 bound for Kyiv, the Ukrainian capital, was in flames before it hit the ground but did not send a distress signal.
A security camera captured the impact — first the predawn darkness, then a series of blinding bursts of light in the distance, followed by a storm of burning debris in the foreground.
The authorities recovered the plane’s “black box” flight data recorders, but they were damaged by the crash and fire, the Iranian report said. That raised the possibility that some of the information stored in them electronically had been destroyed, but in other aviation disasters, investigators have been able to retrieve useful data even from damaged recorders.
The priority “is to find out the causes of the tragedy,” President Volodymyr Zelensky of Ukraine said in a videotaped address released on Thursday. “We will definitely find out the truth. We will conduct a detailed and independent investigation.”
Mr. Zelensky, a comedian with no prior political experience who swept to the presidency in a stunning victory last spring, now faces an excruciating geopolitical balancing act in one of the most challenging crises of his young presidency.
Representatives of the home countries of the plane maker, the airline and the victims ordinarily participate in crash investigations. But in this case, the airplane was manufactured by the United States, which has been involved in a violent dispute with Iran in recent weeks. And Ukraine, already tugged in opposing directions by Russia and the West — and caught in the middle of the impeachment of President Trump — can ill afford to ruffle more feathers in the United States, Kyiv’s most powerful backer.
A team of 45 Ukrainian investigators landed in Tehran on Thursday morning. Mr. Zelensky said he planned to talk to his Iranian counterpart, President Hassan Rouhani, later in the day. The Ukrainian president urged Canada — which lost 63 of its citizens in the crash — to participate in the investigation as well.
The fate of the flight data recorders was unclear. Iranian news media quoted an Iranian aviation official on Wednesday as saying that, contrary to usual practice, the black boxes would not be sent to Boeing.
Iran quickly pointed to possible technical causes for the crash of the plane, Ukraine International Airlines Flight 752.
But the circumstances of the disaster raised suspicions that the airliner may have been attacked, perhaps because it was mistaken for an American warplane; the crash occurred just hours after Iran launched a barrage of ballistic missiles at United States targets in Iraq, with Tehran presumably bracing for possible American retaliation.
Hinting at the possibility of an accidental targeting, Mr. Danilov, the security council secretary, said that the Ukrainian investigative team that arrived in Tehran on Thursday included specialists who had examined the Malaysia Airlines Flight 17 debris in 2014.
But Mr. Zelensky urged Ukrainians not to jump to conclusions.
“This is not a topic for hype, social media likes, sensations and conspiracy theories,” he said in his video address, which was posted to his Facebook page. “We need patience, endurance and wisdom.”
Russia signed a contract in 2005 to sell the sophisticated Tor missile system to Iran, over the objections of American diplomats. Known by the NATO designation Gauntlet, it has a range of just seven miles and is designed to protect the airspace over a small area.
The system is deployed on a tracked vehicle with a crew of four, who can autonomously identify targets and fire the missiles, according to the reference book “Russia’s Arms.” The book, published by Russia’s state weapons exporter, says the system is “designed to destroy,” airplanes, helicopters, drones and missiles at low altitudes.
At Boryspil International Airport near Kyiv, where Flight 752 had been due to land, grieving flight attendants tended to candles set on the floor in front of a makeshift memorial to the nine crew members who had lost their lives. Black-framed portraits of the victims, their names printed on folded sheets of printer paper, rested on a table in front of a pile of flowers several feet high.
A flight attendant named Tatyana, who declined to give her last name because she was not authorized to speak to the news media, said she had visited the memorial on Wednesday evening to pay her respects. She said she had flown the Tehran route before, adding that she always understood that the flight came with additional risks because of the political volatility surrounding Iran.
“Of course there were concerns, risks to those flights,” she said. “We took this responsibility upon ourselves when we joined the airline — to be ready for anything to happen.”
Andrew E. Kramer and Maria Varenikova contributed reporting.