MONTREAL — New evidence indicating that an Iranian surface-to-air missile caused the plane crash this week that killed 63 Canadians is likely to undermine Canada’s already acrimonious relationship with Iran. But the disaster also threatens to damage Ottawa’s crucial but fraught partnership with Washington.

On Thursday, Prime Minister Justin Trudeau of Canada said that his country had intelligence showing that Iran had downed the Ukrainian jetliner, which was carrying 63 Canadians among its some 176 passengers and crew, but he stressed that he wanted a thorough investigation before reaching any conclusions.

“The families of the victims and all Canadians want answers,” he said. “I want answers.”

If it is determined that it was an Iranian missile, Mr. Trudeau will have to grapple with the question of whether the United States, Canada’s most important ally, played a role in provoking the events that ultimately resulted in the loss of Canadians’ lives.

The plane crashed on Wednesday morning, only hours after Iran had launched a barrage of 22 missiles against two military bases housing American troops in Iraq. The Iranian attack was in retaliation for the United States killing of Maj. Gen. Qassim Suleimani, one of the most powerful figures in Iran’s leadership.