Iranian officials have admitted to accidentally shooting down a Ukraine International Airlines passenger jet on Wednesday, killing all 176 on board. In a statement released Saturday, the government blamed “human error” for its military firing the missiles that destroyed the Boeing 737-800, the Associated Press reported.

The Iranian government had previously maintained that engine failure caused the crash, which occurred shortly after the airliner took off from Imam Khomeini International Airport outside Tehran, bound for Kyiv. Many of the passengers were due to make a connecting flight to Toronto—82 Iranians, 57 Canadians and 11 Ukrainians were among the dead.

The crash happened amid heightened tension in the region, just hours after Iran launched ballistic missiles at two military bases in Iraq that house American forces in response to an American drone strike that killed Iran’s top military general, Qasem Soleimani.

A statement from the military on Iranian state media said that the plane was mistaken for a “hostile target,” adding that forces were at the “highest level of readiness” at the time, according to the AP.

“In such a condition, because of human error and in a unintentional way, the flight was hit,” according to the statement. The military apologized, said it was improving its systems to ensure a mistake like this couldn’t happen again and said the people responsible for firing the missiles would be prosecuted.

Iran’s Foreign Minister Javad Zarif blamed the U.S. for Iran’s mistaken decision to shoot down the plane, tweeting: “Human error at time of crisis caused by US adventurism led to disaster.”

He added: “Our profound regrets, apologies and condolences to our people, to the families of all victims, and to other affected nations.”

On Thursday, Canadian Prime Minister Justin Trudeau told reporters that intelligence from multiple sources showed the plane was shot down by an Iranian missile. That assessment was echoed by officials in the U.S. and United Kingdom.

Three U.S. military and intelligence officials told TIME Thursday that they were confident the plane was unintentionally hit by two Iranian anti-aircraft missiles.

“All indications are that the passenger jet was hit by mistake, likely because the Iranian air defenses were on a state of high alert as they launched a barrage of ballistic missiles at U.S. facilities,” a U.S. official said.

Despite the assertions from western leaders, the head of Iran’s Civil Aviation Organization, Ali Abedzadeh, doubled down on his country’s denials on Friday, saying he was “certain” the plane was not struck by a missile.

On Thursday, the U.S. National Transportation Safety Board (NTSB) also announced that it would assist in the investigation at the request of the Iran Civil Aviation Organization, which is leading the probe. Investigators from Ukraine and Canada (at least 63 Canadians were on board the plane, scheduled for a connecting flight to Toronto) are already involved.

Also on Thursday, Boeing said Iran’s civil aviation board had invited the company to look into the downing of the aircraft. The planemaker said earlier that it was “ready to assist in any way needed.”

President Donald Trump dismissed Iran’s initial explanation that a mechanical error caused the crash while speaking at the White House earlier on Thursday, though he did not directly blame Iran for the crash.

“It was flying in a pretty rough neighborhood, and somebody could have made a mistake,” Trump said. “Some people say it was mechanical. I personally don’t think that’s even a question.”

Ukrainian officials have said Thursday that they are considering a missile strike as one several possible causes of the crash. Oleksiy Danilov, secretary of Ukraine’s Security Council, told Ukrainian media that officials had not ruled out a strike, the AP reported.

“A strike by a missile, possibly a Tor missile system, is among the main (theories), as information has surfaced on the internet about elements of a missile being found near the site of the crash,” Danilov said, according to the AP. Iran has refuted the idea that a missile hit the plane, dismissing the accusation as “psychological warfare,” the AP reported.

Airlines including Qantas Airways, Singapore Airlines and Malaysia Air are among the carriers that announced they would be rerouting their services away from the region after the missile salvo and crash. The U.S. Federal Aviation Administration (FAA) had earlier said it was banning civilian aircraft from operating in the airspace over Iraq, Iran, and the waters of the Persian Gulf and the Gulf of Oman amid Iran’s missile barrage into Iraq.

The plane, which crashed shortly after takeoff, was reportedly trying to return to the airport, but the crew had not sent out a call for help, according to an initial report released by Iranian authorities on Thursday.

Iranian officials have said the Boeing 737-800 crashed due to mechanical issues. Initially, the Ukrainian embassy in Iran agreed, blaming engine failure for the crash, but officials later backed away from that statement and said they would refrain from naming the cause of the crash while the investigation continues.

The crash brings renewed attention to Boeing, which continues to deal with the fallout of two deadly crashes of a different 737 model, the 737 MAX, within the past two years. Those incidents left 346 people dead, leading to worldwide grounding of the MAX. Boeing fired its CEO in December amid the continuing controversy.

Here’s what to know about the Ukrainian Boeing crash.

What happened in the plane crash?

The aircraft departed from Imam Khomeini International Airport in Tehran after being delayed for about an hour, according to the Associated Press. After taking off en route to Kiev, the plane did not go above 8,000 feet in the air, the AP reports, citing the flight-tracker FlightRadar24. A statement from Ukraine International Airlines confirms the aircraft ascended to 2,400 meters, just shy of 8,000 feet.

Video verified by the AP and New York Times shows a small light moving quickly up through the air and a large flash following what appears to be a collision in the sky. For several seconds, everything is silent apart from a dog barking. Then, the sounds of an explosion are heard.

Iranian investigators said on Thursday that the plane was trying to return to the airport, and that no radio call for help was made, according to the Associated Press. An initial investigative report cited eyewitnesses who said they saw the plane in flames before it crashed, the AP reported, noting that the crew of another flight in the air were among those who witnessed the blaze. Data from FlightRadar24 shows the aircraft could have crashed within two minutes of taking off.

Qassem Biniaz, spokesperson for Iran’s Road and Transportation Ministry, told state-run news agency IRNA that Iranian authorities believe one of the plane’s engines had caught on fire. The pilot then lost control of the plane.

However, Ukraine International Airlines officials said during a press conference that the crew of the aircraft had been highly trained and that the plane was in good condition. The airline said it has suspended all flights to Tehran until further notice.

“Given the crew’s experience, error probability is minimal,” said Ihor Sosnovsky, vice president of operations for the airline.

The plane crashed southwest of Tehran, scattering debris across farmland that quickly became engulfed in flames. Video and photo footage from the site of the crash show rescue workers sorting through burned belongings and plane fragments.

Read more: Was a Ukrainian Passenger Plane Lost in the Fog of War?

Who are the victims?

The aircraft had 176 people on board, including passengers from around the world. Nine Ukrainian crew members were on the plane, according to Ukraine International Airlines. Ukrainian foreign minister Vadym Prystaiko said 82 Iranians, 63 Canadians and 11 Ukrainians, including the crew members, were on the flight, as well as 10 Swedish nationals, four Afghans, three Germans and three British nationals.

Iranian officials have given slightly different numbers, saying the death toll is 177, with 147 Iranians and two Canadians on board, The New York Times reports.

The AP reports most passengers were flying to Kyiv on their way to other destinations; some were believed to be international students at Canadian universities. Canadian Prime Minister Justin Trudeau said 138 passengers were heading to Canada, according to the AP.

The New York Times reported that other victims included the family of a well-known Iranian writer who had moved to Canada a few years ago, a reservoir engineer for BP, a newlywed couple who were both computer science students and another couple who were engineering professors at the University of Alberta.

The CBC reported that victims came from across six provinces, including British Columbia, Manitoba, Ontario, Quebec, Nova Scotia and Alberta. More than 30 victims came from Alberta, mostly from the Edmonton area, per the CBC, which noted that the region has a thriving Iranian community.

Payman Parseyan, the former president of Edmonton’s Iranian Heritage Society, told the Washington Post that the community is “devastated” after learning that so many victims came from the city.

“Initially, it was three names, and even with three names, we were getting goosebumps because they were our community members,” Parseyan said. “Then it went to 10, and we were in disbelief. As the night progressed, the number got to 27.”

Who is investigating the crash?

Ukraine International Airlines said in a statement that the investigation into the crash will involve the airline, aviation authorities in Ukraine and Iran, Boeing representatives and the National Bureau of Air Accident Investigation of Ukraine.

The U.S. National Transportation Safety Board also said Thursday that it would be investigating the crash.

Although airliners’ black boxes are typically analyzed by their manufacturer following crashes, Ali Abedzadeh, head of Iran’s Civil Aviation Organization, said the aircraft’s flight data recorders would not be sent back to Boeing or the U.S., according to Iranian news agency Mehr.

“We will not give the black box to the manufacturer and the Americans,” Abedzadeh told Mehr, according to the Times.

The Iranian report on Thursday said that both black boxes containing data and cockpit communications from the plane had been recovered, although they were damaged and some parts of their memory had been lost, according to the Associated Press.

Abedzadeh added that the aircraft’s pilots “did not contact the control tower” ahead of the crash and that the flight crew had not reported any technical problems.

Zelensky said that crash investigators from Ukraine had arrived in Iran to assist with the probe, and that he planned to call Iranian President Hassan Rouhani to discuss the incident, according to the Associated Press.

“Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash,” Zelenskiy said. “We will surely find out the truth.”

Read more: The Iran Plane Crash Could Be the Latest in a Long History of Accidental Shoot-Downs

How has Boeing responded?

Early Wednesday, Boeing tweeted that its “heartfelt thoughts” are with those affected by the crash.

“We are in contact with our airline customer and stand by them in this difficult time,” the tweet read. “We are ready to assist in any way needed.”

How have world leaders responded?

Ukrainian President Volodymyr Zelensky returned to Ukraine from a visit to Oman following the crash and has posted frequent updates to Facebook regarding the investigation. In his first post, Zelensky extended condolences to the families and loved ones of the passengers and crew on board.

Trudeau said he was “shocked and saddened” about the crash, which killed 63 Canadians.

“Our government will continue to work closely with its international partners to ensure that this crash is thoroughly investigated, and that Canadians’ questions are answered,” he said in a statement. “Today, I assure all Canadians that their safety and security is our top priority. We also join with the other countries who are mourning the loss of citizens.”

Canada has reached out to Italy to act as a liaison since the country does not have its own diplomatic representation in Iran, according to the Associated Press. Italian Foreign Minister Luigi Di Maio has pledged “every assistance and support” to Canada in their request to protect Canadian interests and “facili(tate) activities of consular assistance,” the AP reported.

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Write to Mahita Gajanan at mahita.gajanan@time.com.