Iranian authorities have said a Ukrainian airliner, which crashed outside Tehran with the loss of all 176 people on board, turned back after suffering a problem, as Ukrainian experts joined the investigation on Thursday.Both Canada and the United States called for a full investigation to determine the cause of Wednesday's crash, which came shortly after Tehran launched missiles at US troops in Iraq in response to the killing of a top Iranian general in a US drone strike in Baghdad.There was no immediate indication that foul play may have caused the Ukraine International Airlines (UIA) plane to go down soon after take-off, and Ukrainian President Volodymyr Zelensky warned against speculating on the crash causes."The plane, which was initially headed west to leave the airport zone, turned right following a problem and was headed back to the airport at the moment of the crash," the Iranian Civil Aviation Organisation said on its website late on Wednesday."The plane disappeared from radar screens the moment it reached 8 000 feet. The pilot sent no radio message about the unusual circumstances."According to eyewitnesses, a fire was seen on board the plane which grew in intensity," the organisation added, reporting the first findings of its investigation into the crash.MUST READ | Speculation is mounting over whether the Ukrainian plane that crashed in Iran was shot down The organisation said it had questioned witnesses both on the ground and on board a second aircraft which was flying above the Ukrainian Boeing 737 as the disaster unfolded.Santa doll in wreckageHeartbreaking details started emerging about the victims, most of them from Iran and Canada.Body bags were lined up on the ground, and the passengers' personal items - including luggage, clothes, a Santa Claus doll and a boxing glove - were scattered in the debris.According to Ukraine, 82 Iranians, 63 Canadians, 10 Swedes, four Afghans, three Germans and three Britons were on board, as well as 11 Ukrainians - including nine crew.About 30 came from the Iranian community around Edmonton, capital of Alberta province in western Canada, where resident Payman Parseyan described the tragedy as "devastating". "Every one of our community members was touched in one way or another," Parseyan told Canada's national broadcaster CBC.Iran's supreme leader Ayatollah Ali Khamenei offered his "sincere condolences" to the bereaved families.No cooperation with USIran's civil aviation chief, Ali Abedzadeh, said Iran would cooperate with Ukraine, but not send the black boxes to the United States, with which it has had no diplomatic relations for four decades.Without naming Iran directly, US Secretary of State Mike Pompeo issued a statement calling for "complete cooperation with any investigation into the cause of the crash".ALSO READ | Ukraine plane crash: Iran says it won't give America aircraft's black boxesAccording to aviation experts, only a handful of countries are capable of analysing black boxes - notably Britain, France, Germany and the United States.Canadian Prime Minister Justin Trudeau said his government would ensure a "thorough investigation" and that "Canadians' questions are answered".The country is home to a large Iranian diaspora, and UIA offers relatively inexpensive flights between Toronto and Tehran, with a layover in Kiev.