Ukrainian investigators believe the flight crew of the Ukrainian jet was killed instantly when a missile exploded next to their cockpit, penetrating the aircraft with shrapnel and sending it hurtling to the ground on fire.

In an interview at the presidential offices in Kyiv, Oleksiy Danilov, the head of Ukraine's National Security and Defence Council, provided CBC News with an in-depth look at the progress of investigation into Flight PS752, which went down near Tehran on Jan. 8.

"You can only imagine what happened there [the cockpit]," said Danilov. "We understand now that the death was instant, unfortunately."

Photos released by Ukraine's presidential office show parts of the aircraft's cockpit, blackened and pierced by small holes, which investigators say came from the missile as it exploded.

They say they believe they know what type of missile was fired at the plane, but have not released those details publicly.

When the three-year-old Boeing 737-800 crashed less than three minutes after takeoff, 176 people were killed.

Fifty-seven Canadians were among the victims and a total of 138 of the passengers were destined for Canada, intending to transit through the Ukrainian capital.

Ukrainian authorities say the plane had been inspected just two days earlier and was in excellent working condition.

Early Saturday morning, Iran made the surprise announcement that one of its elite Revolutionary Guard anti-aircraft batteries had mistakenly fired a missile at the aircraft shortly after it took off.

Watch an 'outraged' Prime Minister Justin Trudeau react to Iran's admission

Prime Minister Justin Trudeau reacts to Iran's admission that it unintentionally shot down passenger jet PS752, says all Canadians are shocked and appalled at this senseless loss of life. 1:01

On Friday, Canada, the United States and Great Britain said, based on intelligence reports, they believed Iran shot the plane down by mistake. Iranian authorities had vehemently denied it until Saturday morning's dramatic about-face.

Nonetheless, Ukrainian investigators said that they concluded within a few hours of arriving in Tehran that a missile was likely responsible for causing the destruction of the Ukraine International Airlines jet.

"It happened within three to three and half hours after our experts started working in Iran," said Danilov. "We became more certain it was a missile."

Iran 'a difficult country' for investigators, Ukrainian official says

But for the next 24 hours, until Iran made its stunning admission, his team had to keep the information to itself, he said.

"We couldn't make statements right away, because it's hard for our experts to work — it's a difficult country."

Ukraine has 45 investigators on the ground in Tehran.

In a televised address Saturday night, Ukraine's President Volodymyr Zelensky said the team now in Iran is made up of seasoned air accident investigators, members of the ministry of defence and the foreign ministry as well as Ukraine International Airlines.

Danilov said they have been working on only two or three hours of sleep and have been sending detailed photographs of their findings back to a second investigative team in Ukraine's capital, Kyiv.

"We were getting visuals in real time and were watching everything going on there," he said.

Holes are seen in the wreckage of Flight PS752 in this photo released by Ukrainian investigators looking into the crash. (Ukrainian Presidential Press Service/Handout via Reuters)

In the middle of the interview with CBC News, a secure phone in the interview room rang and Danilov excused himself to take a call from Zelensky himself.

The topic was the status of Canadian air crash investigators.

Only a few of the expected dozen or so Canadian team members have so far been granted Iranian visas, whereas Iran expedited the arrival of the Ukrainian investigators.

Defining the type of mistake is at centre of investigation

Danilov confirmed that parts of the aircraft recovered from the crash site have been taken to a nearby aircraft hangar where the destroyed plane is being recreated.

Outside investigators have raised concerns that by moving the debris so quickly, valuable clues might have been missed, but Danilov says he doesn't believe that's the case.

Although Iran says the plane was shot down by mistake, Danilov says the question of how they could mistake a commercial aircraft for a threat now looms over the inquiry.

When asked about it in Ottawa on Friday, Prime Minister Justin Trudeau suggested more answers are still required.

"That question is a key part of the investigation," said Danilov. "Why did they make such a decision? Was this a technical mistake, a human mistake or something else."

Oleksiy Danilov, the head of Ukraine's National Security and Defence Council, provided CBC News with an in-depth look at the progress of investigation into Flight PS752. (Pascal Dumont/CBC News)

In a news conference in Tehran on Saturday, representatives of Iran's Revolutionary Guard suggested the decision to fire the missile was made in a split-second by a lone operator.

The IRGC says that person has already been interviewed, but it's not clear if the Ukrainian investigators have been given those details.

Earlier on Saturday, top management with Ukraine International Airlines said they were relieved that the Iranian admission will spare the families — and the company — the agony of a confrontational investigative process.

"In our era of information, it is stupid to even try to cover something up," said UIA vice-president Ihor Sosnovskiy.

The company confirmed that PS752 had taken off on the same flight route many times in the past, and says there was no deviation toward any sensitive military installations, as Iran's military had initially suggested.

Indeed, Sosnovskiy says, not only did several other airlines — including Qatar Airlines — take off safely from Tehran that night, airport operations even continued briefly after the crash.

"There were even a few flights that took off after us. The airport kept working, as if nothing had happened," he said.

Delay didn't factor into crash

While investigators have yet to see or hear the contents of the aircraft's two black boxes, Sosnovskiy says they have been able to listen to the pilots' final conversations with the tower and nothing was amiss.

"In an absolute calm voice, they said they had liftoff," he said. "They received the next permission to continue with the flight and strictly followed the instructions of air traffic control."

The doomed Ukrainian jet was an hour late leaving Tehran, however.

UIA says the holdup was a baggage-related delay, as the pilot was concerned the aircraft was overweight.

Photos of the Ukrainian crew members of Flight PS752 are seen at a memorial at Boryspil Airport in Kyiv. (Corinne Seminoff/CBC News)

Danilov says out of an abundance of safety, the pilot ordered an unusually large number of suitcases — 80 in total — to be taken off the plane.

But he says he believes the flight delay didn't have anything to do with the crash nor would have put the aircraft in a compromised situation.

"It happened because the citizens that were on the aircraft had a lot of baggage," Danilov said.

He said those 80 bags remain in a customs area at Tehran's airport.

In his televised address Saturday, Ukraine's president said he hoped the investigation would now be able to move at a faster pace and the bodies of the victims might soon be returned to their loved ones, possibly by Jan. 19.

"I will return all the deceased to their close ones and relatives. They will able to say goodbye ... in a human way and we will honour them," Zelensky said.