The move came after Western leaders said the plane appeared to have been unintentionally hit by a surface-to-air missile near Tehran hours after Iran launched ballistic missiles at two U.S. bases in Iraq to avenge the killing of its top general in an American airstrike.
Iranian officials have ruled out a missile attack, and Cabinet spokesman Ali Rabiei said late Thursday that such allegations "rub salt on a painful wound" for the families of the victims.
The ballistic missile attack on the bases caused no casualties, raising hopes that the standoff over the killing of Gen. Qassem Soleimani would end relatively peacefully. But Iran has sent mixed signals over whether its retaliation is complete.
If the U.S. or Canada were to present incontrovertible evidence that the plane was shot down by Iran, even if unintentionally, it could have a dramatic impact on public opinion in Iran.
The Iranian public had rallied around the leadership after the killing of Soleimani last week, with hundreds of thousands joining the general's funeral processions in several cities, in an unprecedented display of grief and unity.
But sentiments in Iran are still raw over the government's crackdown on large-scale protests late last year sparked by the worsening economic situation. Several hundred protesters were reported to have been killed in the clampdown.
Those fissures could quickly break open again if public evidence is presented that Iranian authorities were responsible for the deaths of 176 people, mainly Iranians or dual Iranian-Canadian citizens.
The state-run IRNA news agency quoted a Foreign Ministry spokesman as saying Iran "has invited both Ukraine and the Boeing company to participate in the investigations." The spokesman, Abbas Mousavi, said it will also welcome experts from other countries' whose citizens died in the crash.
Iran had initially said it would not allow Boeing to take part in the probe, going against prevailing international norms on crash investigations. It later invited the U.S. accident-investigating agency to take part in the investigation.
The National Transportation Safety Board said late Thursday that it would "evaluate its level of participation," but its role could be limited by U.S. sanctions on Iran. U.S. officials have also expressed concern about sending employees to Iran because of the heightened tensions.
Under rules set by a United Nations aviation organization, the NTSB is entitled to participate because the crash involved a Boeing 737-800 jet that was designed and built in the U.S.
There was no immediate comment from Boeing.
U.S., Canadian and British officials said Thursday it is "highly likely" that Iran shot down the Boeing 737 that crashed near Tehran late Tuesday. U.S. officials said the jetliner might have been mistakenly identified as a threat.
Canadian Prime Minister Justin Trudeau, whose country lost at least 63 citizens in the downing, said "we have intelligence from multiple sources including our allies and our own intelligence."
"The evidence indicates that the plane was shot down by an Iranian surface-to-air missile," he said. U.K. Prime Minister Boris Johnson and Australian Prime Minister Scott Morrison offered similar statements.
Ukraine's President Volodymyr Zelenskiy said "the missile theory is not ruled out, but it has not been confirmed yet."
In a Facebook post, he reiterated his call "on all international partners" — the U.S., Britain and Canada in particular — to share data and evidence relevant to the crash. He also announced plans to discuss the investigation with U.S. Secretary of State Mike Pompeo later on Friday.
Mousavi said Iran asks Canada's prime minister and any other government to "provide any information they have to the investigation committee."
Iranian officials initially said the plane appeared to have crashed because of technical difficulties.
A preliminary Iranian investigative report released Thursday said that the airliner pilots never made a radio call for help and that the aircraft was trying to turn back for the airport when the burning plane went down.
The Iranian report suggested that a sudden emergency struck the Boeing 737, operated by Ukrainian International Airlines, just minutes after taking off from Imam Khomeini International Airport in Tehran early Wednesday.
Ali Abedzadeh, head of Iran's national aviation department, said that by law there is "full coordination" between the country's air defenses and the civil aviation system. He said it is "absolutely impossible" that the armed forces would shoot down a civilian plane.
He said authorities have recovered two black box flight recorders, saying they are "damaged" but readable, which may shed further light on what caused the crash.
Ukraine said Iranian authorities allowed Ukrainian investigators to examine fragments of the plane late Thursday.
"It is too early on in the investigation to reveal specific details," the statement from the Ukraine president's office said. It added that DNA is being collected from relatives of Ukrainians who died in the crash in order to identify the bodies.