The crash of the Ukraine International Airlines (UIA) flight came hours after Tehran had launched a barrage of missiles at bases housing American troops in Iraq, retaliating for the killing of commander Qasem Soleimani in a US strike.

Immediately, theories multiplied arguing that Tehran's air defence system, on high alert for possible US retaliation, may have been triggered by accident.

Western leaders cited intelligence pointing to an accidental strike by Iran. Verified video images later emerged showing a missile striking a plane.

From Wednesday to Friday, Iranian officials repeatedly insisted that the plane had not been shot down. Early on Saturday however, the Islamic republic admitted that the plane had been hit by a short-range missile in what President Hassan Rouhani described as a "catastrophic mistake".

- What happened on January 8? -

The Boeing 737-800NG for UIA's flight PS752 between Tehran and Kiev took off from Imam Khomeini International Airport at 6:12 am local time.

Although there was no distress message from the pilot, the plane had begun to turn back for the airport before crashing at 6:18 am, said the Iranian Civil Aviation Organization. The aircraft came down near Sabashahr on the outskirts of Tehran.

There were 10 departures from the Tehran airport that day before flight PS752, according to the Flight Radar 24 monitoring site. That raised questions as to why air traffic had not been halted given the situation.

All 176 on board the flight PS752 -- mainly Iranian-Canadian dual nationals but also Ukrainians, Afghans, Britons and Swedes -- were killed.

- How did pressure grow? -

Prime Minister Justin Trudeau was the first major leader to publicly state Iran could be to blame. He said Canada had intelligence from multiple sources "including our allies and our own intelligence" that indicated "the plane was shot down by an Iranian surface-to-air missile."

"This may well have been unintentional," Trudeau added.

His language was echoed by other Western leaders including Dutch Premier Mark Rutte, NATO chief Jens Stoltenberg and British Prime Minister Boris Johnson.

US President Donald Trump said he had "suspicions" as "somebody could have made a mistake", and US Secretary of State Mike Pompeo then said the United States believed it was "likely" that an Iranian missile downed the airliner.

The New York Times also said it had verified a video appearing to show a missile hitting a plane at night above the Tehran suburb of Parand, west of the airport.

- What did Iran initially say? -

For three days, Iranian officials refused to countenance any suggestion that one of its missiles had brought down the plane.

The country's civil aviation chief Ali Abedzadeh said that one "thing is for certain, this airplane was not hit by a missile", arguing that it would have exploded immediately if hit by a missile.

Experts questioned that claim, arguing that such missiles explode before coming into contact with the target.

- What does Iran now admit? -

In an announcement early Saturday that took many experts by surprise, Iran admitted it had "unintentionally" shot down the jet after a missile operator mistook the plane for a cruise missile.

Tehran said its systems had been on high alert for American retaliation in the hours after the Iranian strikes on bases housing US troops in Iran.

"He had 10 seconds to decide," the aerospace commander of Iran's Revolutionary Guards Brigadier General Amirali Hajizadeh said of the operator. "He could have decided to strike or not to strike and under such circumstances he took the wrong decision."

Rouhani called it an "unforgivable mistake", vowed that compensation would be paid to the families while those responsible would be prosecuted.

- What kind of missile was used? -

Unverified images posted on social media show the remains of a Russian-made Tor M-1 missile but Tehran has yet to give details on the kind of weaponry used.

Tehran received 29 such air defence systems from Moscow under a $700 million contract signed in 2005, a deal that caused considerable unease in the West.

The Bellingcat open-source analytics website has said the origin of the images is yet to be determined, and the people who captured the images have not come forward.

- What does Ukraine want now? -

Ukrainian President Volodymyr Zelensky, initially wary of blaming Tehran, has toughened his line since Tehran's admission.

Zelensky, who will speak to Rouhani later Saturday, called for Tehran to punish those responsible, pay compensation, apologise and give "total access" to Ukrainian aviation experts.

Despite the three days of denial from Tehran, some have compared its reaction favourably to that from Moscow over the crash of the Malaysia Airlines flight shot down over eastern Ukraine in 2014.

Although the Dutch-led Joint Investigation Team says a Russian-made BUK missile fired by pro-Russian separatists was to blame, Russia still denies any involvement.

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Malaysia Airlines

THE NEW YORK TIMES COMPANY