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The mystery has deepened over the airliner crash in Iran which killed 176 people after an initial report claimed some of the “black box” memory had been lost.

Investigators from Iran’s Civil Aviation Organisation also said the crew of Ukraine International Airlines flight PS752 were trying to turn the aircraft back minutes after take-off from Tehran.

The pilot never made a call for help, the report added, suggesting catastrophe struck the plane extremely quickly.

Eyewitnesses, including the crew of another passing flight, described seeing the plane engulfed in flames as it came down in the morning darkness.

Iran: Tehran Plane Crash - In pictures 10 show all Iran: Tehran Plane Crash - In pictures 1/10 via Reuters 2/10 AP 3/10 AFP via Getty Images 4/10 People stand near the wreckage after a Ukrainian plane crashed near Imam Khomeini airport in Tehran ISNA/AFP via Getty Images 5/10 AP 6/10 AP 7/10 via Reuters 8/10 AP 9/10 Rescuers check the debris following the plane crash via Reuters 10/10 Part of the wreckage from Ukrainian plane that crashed in Tehran shortly after take-off via Reuters 1/10 via Reuters 2/10 AP 3/10 AFP via Getty Images 4/10 People stand near the wreckage after a Ukrainian plane crashed near Imam Khomeini airport in Tehran ISNA/AFP via Getty Images 5/10 AP 6/10 AP 7/10 via Reuters 8/10 AP 9/10 Rescuers check the debris following the plane crash via Reuters 10/10 Part of the wreckage from Ukrainian plane that crashed in Tehran shortly after take-off via Reuters

The plane suffered a massive explosion, either before or when it hit the ground, which was likely because the aircraft had been fully loaded with fuel for the flight to Kiev, Ukraine’s capital.

The report claimed that both of the so-called “black boxes” containing data and cockpit communications from the Boeing 737 had been recovered, though they had suffered damage and some parts of their memory was lost.

It also said that investigators have initially ruled out laser or electromagnetic interference as causing the crash, which happened at 6.12am local time soon after the plane had left Imam Khomeini International Airport.

The crash came just hours after Iran carried out missile strikes on two air bases housing US forces in Iraq.

However there is no evidence the two events are linked

The Iranian and Ukranian authorities initially suggested that engine failure may have caused the tragedy. But aviation experts and the carrier cast doubt on whether this was likely, especially as the plane was just three years old.

David Learmount, safety editor of Flight Global, told BBC Radio 4’s Today programme: “It’s incredibly rare for a modern aircraft to have a technical failure ... which causes a conflagration [major fire] before the aircraft has even hit the ground.”

With just one engine, even if it was not functioning fully, the plane could have flown, he added.

“There never has been a catastrophic engine failure which was so bad that the aircraft was completely uncontrollable and caught fire,” he stressed.

“You have to look at the possibility that it was hit by a missile or that there was a bomb on board.”

US and Canadian intelligence officials, though, said there was no immediate evidence it was shot down.

Theories include that the plane could have collided with a military drone or been hit by accident by a missile having climbed to just under 8,000 feet.

Suspicions have been fuelled by Iran saying it would not hand over the black boxes to Boeing or the US.

Under global aviation rules Iran has the right to lead the investigation, but manufacturers are typically involved, and there are doubts whether it would have the expertise to analyse all the data.

Three British passengers died in the disaster: Sam Zokaei, 42, an engineer for BP from Richmond, Saeed Tahmasebi Khademsadi, 35, from Brentford, who worked as an engineer for global construction company Laing O’Rourke, and Mohammad Reza Kadkhoda Zadeh, 40, who owned a dry-cleaning business in Brighton.

The plane was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, at least 63 Canadians and 11 Ukrainians, according to officials.

Ukrainian president Volodymyr Zelensky said crash investigators from his country had arrived in Iran to assist in the probe. He also said he planned to call Iranian president Hassan Rouhani about the crash and investigation.

Canadian prime minister Justin Trudeau said 138 of the passengers were bound for his country.

“Know that all Canadians are grieving with you,” he said, addressing the victims’ families.

Among the Canadians killed were a number of university students and staff, including engineering professors Pedram Mousavi and Mojgan Daneshmand, a married couple from the University of Alberta, along with their two daughters, Daria, 14, and Dorina, nine.

Also killed were Evin Arsalani, 30, her husband Hiva Molani, 38, and their one-year-old daughter Kurdia, from Ajax, Ontario. They were returning home after spending three weeks in Iran to attend a wedding.

Dentist Parisa Eghbalian, 42, and her nine-year-old daughter, Reera Esmaeilion, also died.

They were flying home having attended Mrs Eghbalian’s sister’s engagement party in Iran.

Her husband Hamed Esmaeilion said: “She was my love for 20 years. She was a wonderful woman and she was very, very intelligent.”

Ukrainian officials initially agreed with Iranian suspicions that the plane was brought down by mechanical trouble but later declined to offer a cause while the investigation is going on.

While the cause of the tragedy remains unknown, the disaster could further damage Boeing’s reputation, which has been battered by the furore over two deadly crashes involving a different model of the Boeing jet, the much-newer 737 Max, which has been grounded for nearly 10 months.

The company’s CEO was fired last month.