An Iranian surface-to-air missile brought down a passenger plane bound for Kyiv, Ukraine, shortly after takeoff from Tehran this week, killing all 176 people on board, officials from the U.S., Canada and the U.K. said Thursday, citing new intelligence. "This may well have been unintentional," Canadian Prime Minister Justin Trudeau told a news conference, urging the Iranian government to ensure a "thorough investigation" of the crash. Ukraine International Airlines Flight 752 crashed Wednesday morning, after Iran had launched missile attacks on bases in Iraq that house U.S. and coalition forces, prompting the theory that it was struck by a stray missile, a theory that U.S. and Canadian government officials on Thursday said was likely, based on new information. Sixty-three Canadians were among the victims in the crash of the flight, which lost contact with air traffic controllers five minutes after takeoff from Tehran on Wednesday morning, officials said. "The news will undoubtedly come as a shock to the families who are already grieving in the face of this unspeakable tragedy," Trudeau said. "We have intelligence from multiple sources, including our allies' and our own intelligence" that indicates the plane was shot down by an Iranian missile. The reports came after President Donald Trump said he did not believe mechanical failure caused the crash. "It was flying in a pretty rough neighborhood," Trump said. "Someone could have made a mistake." Spy satellite imagery suggested the Ukraine-bound Boeing passenger airliner was shot down by an Iranian missile NBC News reported earlier Thursday. Iran's head of civil aviation agency dismissed the missile reports as "illogical rumors." Iranian officials had originally suggested that the Boeing 737-800 crashed because of a technical problem.

'Need for a thorough investigation'

Iran's civil aviation authority, which did not immediately respond to requests for comment from CNBC, said earlier Thursday that the plane departed Tehran's Imam Khomeini airport at 6:13 a.m. local time on Wednesday and lost communication with air traffic controllers five minutes later. Under 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. Fraught relations between the U.S. and Iran could also complicate the U.S. government's willingness to send personnel to the country. The National Transportation Safety Board on Thursday evening said it appointed a designee for the process, but it wasn't immediately clear when the person or team would be able to travel. Some airlines in Europe called off flights to Iran in the wake of the crash. tweet An Iranian official told local news media on Wednesday that Iran did not plan to share the black boxes with the U.S but Iran's aviation agency said it was conducting its investigation under international law and that it has "provided all relevant countries with the necessary information since the accident." Iran may send the recovered black boxes, which contain flight data and cockpit voice recordings, abroad for analysis. Iranian aviation officials said the boxes that were recovered from the crash site were damaged by fire. Ali Abedzadeh, head of Iran's aviation authority, said that Iran and Ukraine can download the data from the black boxes but that it was open to sharing the information with specialists in other nations. "This new information reinforces the need for a thorough investigation of this matter," Trudeau said.

"I have a feeling that — it's just some very terrible, something very terrible happened, very devastating," Trump added. Most of the passengers on the flight were from Iran and Canada. Other passengers came from countries including Sweden and Ukraine. Newsweek noted that images that began circulating on Wednesday show what looked to be fragments of a Tor M-1 missile that were said to be found in a Tehran suburb. Sen. Jim Inhofe, R-Okla., said, "It sounds pretty convincing that the plane that was shot down ... by Iranians." Asked what the U.S. response should be if true, Inhofe paused and then said, "I don't want to interfere with the decisions that are now being made by the president." "I'm quite sure I'll be talking to him because we talk on a regular basis about things like that."

Aircraft parts from the wreckage of a Boeing Co. 737-800 aircraft, operated by Ukraine International Airlines, which crashed shortly after takeoff lie on the ground near Shahedshahr, Iran, on Wednesday, Jan. 8, 2020. Ali Mohammadi/Bloomberg via Getty Images

Sen. Lindsey Graham, R-S.C., said that if Iranian missiles had shot down the plane "it would be an outrage." "What I would do if I were the president would be to reach out to the Canadian people and the prime minister and try to rally the world around the idea that we shouldn't accept 40 more years of state-sponsored terrorism," Graham said. Abedzadeh said in a statement that eyewitnesses had reported that the Kyiv-bound plane was on fire immediately before the crash. Aviation safety experts, including former government crash investigators, told CNBC that the sudden loss of communication and lack of a distress signal were highly suspicious signs. The crash occurred hours after Iran launched retaliatory missile strikes on U.S. positions in Iraq for the killing of Gen. Qasem Soleimani. The timing prompted speculation that a stray Iranian missile may have downed the passenger plane. Determining the cause of plane crashes can take more than a year.

Rescue workers recover bodies from the wreckage of a Boeing Co. 737-800 aircraft, operated by Ukraine International Airlines, which crashed shortly after takeoff near Shahedshahr, Iran, on Wednesday, Jan. 8, 2020. Ali Mohammadi | Bloomberg | Getty Images