The leaders of Australia, Canada and the U.K. all believe there is evidence to suggest an Iranian missile brought down a Ukrainian passenger plane earlier this week, with analysts expressing concern that intensifying international pressure against Tehran will soon trigger a “diplomatic crisis.”Ukrainian International Airlines Flight 752 crashed on Wednesday morning, killing all 176 people on board. Most of the passengers on the flight were from Iran and Canada. Other passengers came from countries including Sweden and Ukraine.The crash occurred just a few hours after Iran launched a ballistic attack on military bases housing U.S. troops in Iraq, prompting a theory that the Kyiv-bound passenger plane may have been unintentionally struck by a stray missile.Government officials from Australia, Britain, Canada and the U.S. have since said they believe this theory is likely, based on new information.Iran has denied the plane was brought down by a missile.”If the preliminary assessments prove accurate, the diplomatic fallout for Iran will be significant in the short term,” analysts at Eurasia Group said in a research note published Thursday.Search and rescue works are conducted at site after a Boeing 737 plane belonging to a Ukrainian airline crashed near Imam Khomeini Airport in Iran just after takeoff with 180 passengers on board in Tehran, Iran on January 08, 2020.Fatemeh Bahrami | Anadolu Agency | Getty ImagesAnalysts at the political risk consultancy suggested Tehran seemed to earn a “good deal of international sympathy and solidarity” after the U.S. killed Iranian military commander Qasem Soleimani last week — with many “quietly grateful” the Islamic Republic did not respond more forcefully.But, “the shootdown will jeopardize that diplomatic goodwill, especially if the evidence proves overwhelming and Iran continues to stridently deny it.”‘Pretty rough neighborhood’Speaking to reporters at a press conference on Friday, Iran’s Civil Aviation Authority head, Ali Abedzadeh, said Tehran wanted to download black box recordings itself.It if needed help to decode the messages — which could reportedly take one or two months — Iran could ask for help from Russia, Canada, France or Ukraine, Abedzadeh said.Sixty-three Canadians were among the victims in the crash of the flight, which lost contact with air traffic controllers five minutes after take-off from Tehran on Wednesday morning, officials said.Canadian Prime Minister Justin Trudeau told a news conference on Thursday that he had “intelligence from multiple sources including our allies and our own intelligence.””The evidence indicates that the plane was shot down by an Iranian surface-to-air missile,” Trudeau added. British Prime Minister Boris Johnson and Australian Prime Minister also issued similar statements.Canada severed diplomatic ties with Iran in 2012 and, despite pledges to re-engage, Trudeau has not yet sought to revive international relations.Ukrainian President Volodymyr Zelenskiy said the theory that a stray missile may have struck Flight 752 could not be confirmed yet.Meanwhile, President Donald Trump has said he does not believe a mechanical failure caused the crash. “It was flying in a pretty rough neighborhood,” Trump said. “Someone could have made a mistake.”‘One or two years’ to complete flight investigationUnder international law, the country in which the crash occurs controls the investigation, and because the plane was U.S.-made, federal investigators and Boeing, the plane’s manufacturer, would normally be involved.However, fraught relations between the U.S. and Iran could complicate the U.S. government’s willingness to send personnel to the country.”It is quite tragic that these people were all murdered but, if anything, it creates this image of the Iranian military being trigger happy and not being overly concerned about deaths of civilians,” Frank Lavin, former U.S. under secretary of commerce for international trade, told CNBC’s “Capital Connection” on Friday.”So, if anything, I think it is going to subdue the Iranian military for the short run,” Lavin said.An Iranian official told Reuters on Friday that it could take “one or two years” to complete an investigation into the flight.”Iran will face intense political and economic pressure to cooperate with international aviation authorities, and we expect Tehran will eventually accept Western investigators,” analysts at Eurasia Group said.— CNBC’s Leslie Josephs and Dan Mangan contributed to this report.