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Prime Minister Justin Trudeau says a total of 138 passengers aboard the plane that were connecting to Canada.

“All had so much potential, so much life ahead of them,” Trudeau told reporters, adding Canada expected to have a role in the crash investigation.

Flags flew at half-mast across the country, including at Parliament in Ottawa, and vigils were scheduled in several Canadian cities.

The flight was a popular transit route for Canadians traveling to Iran, in the absence of direct flights, and carried many students and academics heading home from the holidays. Canada broke off diplomatic relations with Iran in 2012.

Iran’s Disaster Mitigation and Management Organization said early assessments indicate the cause was a technical issue; the transport ministry suggested an engine fire was to blame.

The initial assessment of Western intelligence agencies is that the airliner was not brought down by a missile, a Canadian security source told Reuters. The source, who declined to be identified, said the agencies believed the Boeing 737 plane had suffered a technical malfunction.

Other authorities didn’t initially follow Iran in pointing to a fire or technical issue. The Ukrainian embassy in Tehran amended an earlier statement in which it had ruled out terrorism, instead making no comment on possible causes.

The varying responses came as peculation swirled that the jet may have been downed by a stray missile following Iranian strikes against U.S. bases in Iraq, or in a terrorist attack. Global Affairs Canada has warned Canadian citizens against all non-essential travel to Iran.