While the cause of the crash remains unclear, some aviation experts said that what was known indicated that the plane could have been attacked. Investigators should have that possibility “at the top of their agenda,” said Peter Goelz, a former managing director of the National Transportation Safety Board in the United States.

What did Ukraine and Iran say about the crash?

Early statements from both countries were somewhat contradictory.

Qassem Biniaz, an official at the Iranian Ministry of Roads and Urban Development, told the Islamic Republic News Agency, the government’s official news agency, that an engine had caught fire and the pilot was unable to regain control.

Iranian news organizations tied to the government initially referred to technical problems with the plane, without providing details or evidence. Ukraine’s Embassy in Iran initially issued a statement ruling out terrorism or a rocket attack as a cause of the crash.

But Ukraine’s statement was later removed from the embassy’s website and replaced by one saying it was too early to draw any conclusions. The head of Iran’s Civil Aviation Organization, Ali Abedzadeh, told the semiofficial Mehr News Agency that so far there was no evidence of technical problems.

A spokesman for Iran’s armed forces, Abolfazl Shekarchi, said the crash was not a result of any military action.

“They are spreading propaganda that the Ukrainian flight was targeted,” the Iranian news media quoted Mr. Shekarchi as saying. “This is ridiculous. Most of the passengers on this flight were our valued young Iranian men and women. Whatever we do, we do it for the protection and defense of our country and our people.”

Prime Minister Justin Trudeau of Canada said in a statement that he was “shocked and saddened” that the crash had “claimed the lives of 176 people, including 63 Canadians.”