Iranian civil aviation officials said Thursday that preliminary findings from an investigation into Wednesday morning's crash of Ukraine International Airlines Flight 752 outside Tehran showed that the plane had started to return to the airport minutes after takeoff but "no radio messages were received from the pilot regarding unusual situations" before the airliner went down, killing all 176 people on board. Iran's military said it had no role in the plane's fiery crash, hours after it fired more than a dozen ballistic missiles toward U.S. forces in Iraq. Iran initially blamed the crash on mechanical issues.

Ukraine, which originally ruled out terrorism or a rocket attack, is now keeping an open mind. Ukrainian crash investigators arrived in Tehran on Thursday.

Flight 752, a Boeing 737-800 that had been serviced Monday, took off at 6:12 a.m., after nearly an hour's delay, and everything appeared normal until it reached nearly 8,000 feet, at which point it stopped transmitting flight data. "Eyewitnesses, including the crew of another flight passing above it, described seeing the plane engulfed in flames before crashing at 6:18 a.m.," The Associated Press reports, citing Iran's preliminary report. "The crash caused a massive explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel for the flight to Kyiv."

"All possible versions of what occurred must be examined," said Ukrainian President Volodymyr Zelensky. "Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash." Oleksiy Danilov, secretary of Ukraine's Security Council, said the working theories include a drone hitting the plane, a terrorist attack, engine malfunction, and "a strike by a missile, possibly a Tor missile system," citing internet reports. Russia delivered 29 Tor-M1 missiles to Iran in 2007, AP notes.

Ukraine said the flight's 167 passengers included 82 Iranians, 63 Canadians, and 11 Ukrainians. Canadian Prime Minister Justin Trudeau did not rule out a missile strike but said it's too early to draw any conclusions.

Trudeau said 138 of the passengers were headed to Canada via Kyiv, many of them believed to be international students. Peter Weber