A Ukrainian airliner that crashed outside of Tehran was on fire and trying to turn back but its crew never made a radio call for help, Iranian investigators have said.The Ukrainian International Airlines Boeing 737-800, flying to Kiev and carrying mostly Iranians and Iranian-Canadians, crashed shortly after taking off on Wednesday from Tehran's Imam Khomeini airport, killing all 176 on board.More:Iran-US tensions likely to hamper Ukraine plane crash probeUkraine does not rule out attack as cause of plane crash in Iran'No survivors': Ukrainian jet crashes in Iran with 176 on boardEyewitnesses, including the crew of another passing flight, described seeing the plane engulfed in flames before crashing, the investigators' report said. The crash caused a massive explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel for the four-hour flight."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. "The plane disappeared from radar screens the moment it reached 8 000 feet (2 400 metres). The pilot sent no radio message about the unusual circumstances."Ukrainian President Volodymyr Zelenskyy, meanwhile, said crash investigators from his country had arrived in Iran to assist in the probe. "Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash," Zelenskyy said. "We will surely find out the truth." He also declared Thursday a day of national mourning.Both flight recorders - known as "black boxes" - from the three-year-old jet have been found and are being investigated. Iranian Transport Minister Mohammed Eslami said they would be handed over to Ukraine. But the country that manufactures the aircraft is also supposed to have representatives involved in such a probe. In theory, this means that the US National Transportation Safety Board (NTSB), which is the body charged with investigating air accidents, would be involved. The aeroplane's manufacturer, Boeing, is based in the US, and investigators would likely rely on experts from the manufacturer. However, the head of Iran's Civil Aviation Organisation, Ali Abedzadeh, said while the Ukrainians were free to participate in the investigation into the crash, "we will not give the black boxes to the manufacturer [Boeing] and the Americans," according to Iran's Mehr news agency on Wednesday.A Canadian security source told Reuters news agency there was evidence one of the jet's engines had overheated.The crash happened hours after Iran launched missile attacks on US-led forces in Iraq, in retaliation to the US assassination of a top Iranian commander, leading some to speculate that the plane may have been hit.The initial assessment of Western intelligence agencies was that the plane had suffered a technical malfunction and had not been brought down by a missile, five security sources - three Americans, one European and the Canadian - who asked not to be named, told Reuters.Bodies and body parts recovered from the site of the crash have been taken to the coroner's office for identification, the investigators' initial report said.Smouldering debris, including shoes and clothes, were strewn across a field where the plane crashed on Wednesday, where rescue workers in face masks laid out scores of body bags.Onboard were 146 Iranians, 10 Afghans, 11 Ukrainians, five Canadians and four Swedes, the report said, but noted some may have held citizenship of other countries.Ukrainian authorities have said those on board included 82 Iranians, 63 Canadians, and 11 Ukrainians.Canadian Prime Minister Justin Trudeau said 138 of the passengers were bound for Canada. The flight also included a family of four as well as newlyweds. The manifest listed several teenagers and children, some as young as one or two.The crash ranked among the worst losses of life for Canadians in an aviation disaster. The flag over Parliament in Ottawa was lowered to half-staff, and Trudeau vowed to get to the bottom of the disaster."Know that all Canadians are grieving with you," he said, addressing the victims' families.