While Iran continues to deny evidence cited by Canada and the U.S. that a surface-to-air missile downed Flight PS752, the Ukrainian aircraft's black boxes could provide some crucial clues around the cause of the crash.
An aircraft's black boxes include two components: the flight data recorder and cockpit voice recorder.
Iranian state television showed footage on Friday purportedly of the two black boxes recovered from the site outside Tehran where the Ukrainian International Airlines plane crashed shortly after takeoff, Reuters reported.
The footage, posted online by state TV, showed two devices inside a wooden crate that the news report said were the cockpit voice recorder and the flight data recorder.
Both black boxes are damaged, but their memory can be downloaded and examined, the report said.
The wooden crate was reportedly opened at the Iran Civil Aviation Organization.
WATCH: Iranian TV purportedly shows Ukraine airliner's black boxes
The flight data recorder tracks measurements such as air speed, altitude, heading (bearing) and engine thrust while the cockpit voice recorder records all the communication between crew members, as well as between the crew and air traffic control, and the ambient sound in the cockpit.
"If it's an operational-type accident — operational meaning pilot issues and so on, nothing wrong with the aircraft — then the flight recorders are very, very good at telling you exactly what happened," said Mike Poole, a former investigator with the Transportation Safety Board of Canada and an expert on flight recorders.
"If it's a technical problem with the aircraft, then the flight recorder is [also] very good at telling you there's a technical problem. But pinpointing it usually requires the physical wreckage."
And when it comes to figuring out whether a plane was downed by an outside force, such as a projectile, the recorders can offer some indirect evidence of that.
Signature of a perfectly functioning airplane
"If you're shot down, the signature is typically a perfectly functioning airplane and perfectly normal operations that all of a sudden stops," Poole said. "That doesn't necessarily [mean] it was shot down, but it says whatever happened was instantaneous."
Of the 176 victims killed on board the Ukraine International Airlines flight, 57 were Canadian citizens and a total of 138 were ultimately bound for Canada.
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On Thursday, Prime Minister Justin Trudeau announced that intelligence from multiple sources, including Canada, has indicated Flight PS752 was shot down by an Iranian surface-to-air-missile, perhaps unintentionally.
Iran has denied the allegations.
If true, it would mean the plane met the same fate as Malaysia Airlines Flight MH17, which was shot down over territory held by pro-Russia separatists in Eastern Ukraine in July 2014, killing 296 people, including one Canadian.
In its report into that crash, the Dutch Safety Board concluded the plane was downed by the detonation of a warhead launched by a surface-to-air missile system. While forensic chemical analysis on the wreckage helped make that determination, investigators also used some of the evidence gathered from the flight data recorder and cockpit voice recorder.
One piece of evidence that came from the cockpit voice recorder was a 2.3 millisecond sound peak — a noise, it was concluded, that originated from outside the airplane.
"Normally, when an aircraft is hit by a projectile like that … it can even be heard, because there's a sudden decompression, and most of that is captured in the cockpit voice recorder," said Daniel Adjekum, an assistant professor of aviation at the University of North Dakota.
No alerts or warnings
Investigators also found the MH17 data recorders had stopped abruptly — and they confirmed the normal functioning of the airplane's engines and systems before the crash; no warning failures or discrepancies were recorded.
Nor were there any alerts or warnings of system malfunction heard in MH17's cockpit voice recorders; communication between flight crew members gave no indication of any malfunction or emergency prior to the crash.
"This will be clues to the investigators that whatever happened was sudden — it was instantaneous in a way that their recordings were abruptly stopped," Adjekum said. "Those are clues that, most likely, it was hit by a projectile."
Questions have already been raised over potential access to the black boxes of Flight PS752.
Based on international aviation regulations, Iran has authority over the crash probe since it occurred in their territory.
While representatives of the plane's manufacturer are often involved in the investigation of the crash and analysis of the flight recorders, in this case, the plane was manufactured by U.S.-based Boeing. But with the ongoing standoff between the U.S. and Iran, the country's aviation authority has said it would not send the black boxes to the American company.
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(Iran has said that Ukranian officials can be present, however, as well as Canadians, albeit in a limited capacity.)
Iranian state media reported on Friday that U.S., Canadian and French representatives are to travel to Tehran to attend meetings for the Iran-led investigation into the crash.
"As soon as they will arrive they will attend the meetings to investigate reasons for the crash," IRNA reported.
It is unlikely that Iran has the technology needed to access the information from the black boxes, with officials already saying they may need to outsource the recorders to outside experts.
Adjekum believes the best way forward is to get a third-party country involved — one that has relations with Iran and the U.S., as well as the technical capability to retrieve the data.
"France might be my best bet," he said. "Send it out to France. [American] NTSB investigators can also travel to France, a third-party country, and they can all be there when the data … is read out.
"And it will satisfy everybody in terms of transparency and openness."
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U.S. will assist investigation
In 2018, the U.S. re-imposed economic and arms-trade sanctions on Iran following the Trump administration's exit from the Iran nuclear deal.
These target critical sectors of Iran's economy, such as the nation's ability to export oil, energy, shipping and shipbuilding, and financial sectors.
Iran is also one of the seven mostly Muslim countries subject to a travel ban by the U.S.
On Friday, the U.S. imposed more sanctions on Iran following this week's missile strikes by the Islamic Republic on U.S. bases in Iraq.
However, U.S. Treasury Secretary Steven Mnuchin said on Friday the department will grant sanction waivers to allow Americans or anyone else to participate in the investigation of Wednesday's crash.
Under U.S. sanctions law, the U.S. Treasury's Office of Foreign Assets Control (OFAC) must grant approval for U.S. investigators and Boeing to participate and potentially travel to Iran.
WATCH: Pompeo liaising with Canadian counterpart on plane crash
Boeing said Friday it was working with the Federal Aviation Administration (FAA) and National Transportation Safety Board (NTSB) "on the necessary applications and approvals from OFAC for the appropriate export licenses."
The NTSB said late Thursday it had agreed to be an accredited representative to the investigation of the crash at Iran's invitation.
"The Treasury will issue waivers for anybody, whether its Americans or others, to help facilitate the investigation," Mnuchin said at a White House press conference.
U.S. Secretary of State Mike Pompeo also said Friday in the joint press announcement that it was "likely that plane was shot down by an Iranian missile."
Reuters and other media reported Thursday that U.S. officials had reached that assessment based on a review of satellite data.
Information provided by the U.S. will help in the investigation of the crash, Zelensky said on Friday, after a phone call with Pompeo.
Ukraine earlier said it had received data from the U.S. that would be processed by experts, without elaborating on the nature of the data or what it signified.
"Grateful for the condolences of the American people & valuable support of the U.S. in investigating the causes of the plane crash," Zelensky wrote on Twitter. "Information obtained from the U.S. will assist in the investigation."
In a separate statement, Zelensky's office said Pompeo would visit Ukraine later in January.
On Friday, a Russian deputy foreign minister said that Moscow currently sees no grounds to blame Iran for the crash of the airliner near Tehran, contradicting statements by Canada and the U.S., according to local media.
Sergei Ryabkov, who is one of several Russian deputy foreign ministers, called on senior world officials to refrain from public statements until more details are known, TASS media agency reported.
While Russia denies the claims, Dutch foreign minister Stef Blok said on Friday that it "was very likely" that an Iranian missile shot down Flight PS752. The EU's next steps would depend on how Tehran reacts to the results of an investigation, Blok said.
Asked whether Iran should be sanctioned, Blok said that "it depends on the Iranian reaction [to the outcome of independent analysis] on what should be the next steps."