The mixed messaging began almost immediately after a Ukrainian passenger jet crashed near Tehran early Wednesday, killing all 176 people on board.

Iranian state media said the crash appeared to be the result of a malfunction. The Ukrainian Embassy in Tehran posted a similar assertion — then promptly deleted the statement. Now, U.S. and Canadian officials have said they have reason to believe that the plane was shot down by an Iranian air defense system — possibly by accident.

On Thursday, U.S. defense officials speaking on the condition of anonymity to discuss the matter publicly, told The Washington Post that U.S. authorities believe the plane was hit by a missile fired from an SA-15 launcher.

Here are some key answers to questions about the missile system and the crash, which occurred several hours after Iran targeted two Iraqi bases housing U.S. troops, in retaliation for the U.S. killing of Iranian military commander Maj. Gen. Qasem Soleimani on Jan. 3 in Baghdad.

What is the SA-15 air defense system?

It is a Russian-made air defense system that allows its operators to shoot down aircraft at relatively close range. It is also known as the Tor air defense.

The SA-15 can shoot down aircraft, precision weapons and guided munitions. It is capable of tracking targets as far as 15.5 miles away, according to military documents, and can intercept targets as high as 32,800 feet.

It is designed to defend against objects flying at medium to very low altitudes and can track up to 48 targets at once, the documents show, with an ability to fire two missiles nearly simultaneously if necessary.

Will Mackenzie, a research associate in the defense program at the Center for New American Security, said the SA-15 “is an older platform” that is “primarily made for short-range air defense, and it’s a mobile system so it can be moved around.”

Michael Duitsman, research associate at the Center for Non-Proliferation Studies at the Middlebury Institute of International Studies, said the system can shoot targets up to about 7.5 miles away.

If a missile was launched, it may have been detected by the Space-Based Infrared System, the U.S. military’s network of satellites designed as an early warning system for ballistic missile launches and tactical missiles. The system can also detect infrared activity from lower-tier events with an infrared signature, such as fires and plane crashes.

Missiles fired from the SA-15 are designed to pepper targets with shrapnel when it explodes, which sends tumbling shards of metal into the fuselages of aircraft and drones.

On Jan. 11, Iranian officials said the military brought down UIA 752 and said the investigation into who was responsible for the mistake must continue. (The Washington Post)

How did Iran obtain this system?

Russian officials announced in 2007 that they had delivered the Tor M-1 system, also known as the SA-15, to Tehran. The Guardian reported that year that Iran had obtained 29 of the systems under the terms of a $700 million contract.

Duitsman said U.S. officials were likely concerned when Iran obtained the SA-15 because “it was a very large improvement over some of the other systems Iran had at the time.”

Trump said someone in Iran could have “made a mistake.” Is that possible?

Observers have noted that Iranian forces may have been on high alert for a U.S. response to missile attacks on bases in Iraq around the same time as the crash.

“If there is an air defense battery and they are worried about an incoming aircraft or missile, they are operating on a hair trigger,” said Michael Spirtas, an associate director of the Rand International Security and Defense Policy Center and a former Pentagon official focused on U.S. defense strategy and air defense. “They are ostensibly defending something important to their leadership; they’d be pretty jumpy. That decision not to fire is a hard one to make.”

The SA-15 does not track threats and fire autonomously. The operator must identify the target on a screen and then fire the missile, Reuters reported.

Duitsman said that if Iran did launch a missile that struck the plane, he believes it would be a “complete accident.”

“The entire event is just so bizarre,” Duitsman said. It is hard to imagine, he said, “that the Iranian missile crew weren’t aware of what was happening, and that they were letting airliners take off when they were on alert expecting an American retaliation.”

What are officials saying?

On Thursday, President Trump said that it was possible that “somebody could’ve made a mistake on the other side.”

“Some people say it was mechanical,” he said. “Personally, I don’t think that’s even a question.”

Later in the day, Canadian Prime Minister Justin Trudeau said “evidence indicates the plane was shot down by an Iranian surface-to-air missile.”

“This may well have been unintentional,” he added. More than 60 of the victims were Canadian citizens.

Ukrainian President Volodymyr Zelensky has asked leaders to share their intelligence with him, but he has urged caution over drawing conclusions while the investigation is still underway.

Ali Abedzadeh, head of Iran’s Civil Aviation Organization, said in a news conference that he was “certain that no missiles hit the aircraft.”

Ukraine is sending a team of 45 personnel to Iran, including experts who helped with the investigation into the July 2014 Malaysia Airlines Flight 17 crash and specialists responsible for taking home the bodies of 11 Ukrainians who were on board that flight.

As The Post reported, Oleksiy Danilov, secretary of Ukraine’s National Security and Defense Council, posted on Facebook that his team would like to look for evidence the plane was possibly taken down by a missile.

Although it was a Boeing plane, it could be complicated for U.S. officials to participate in investigations because of U.S. sanctions in place against Iran.

Dalton Bennett contributed to this report.

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