The head of Ukraine's investigation team has said that evidence from the passengers on board flight PS752 suggests that something had happened to the plane even before it was hit by the missiles fired at it by Iran's Revolutionary Guard shortly after it took off from the Tehran airport.

Alexander Ruvin, Director of the Kyiv Research Institute of Forensics, says evidence shows that passengers were out of their seats before the two missiles hit it shortly after taking off from Tehran's International Imam Khomeini Airport on January 8, Radio Farda reports.

In an interview with Glavcom on May 19 Ruvin said at the time of the crash the plane had not reached the altitude of 8,000 meters when seat belts are allowed to be unfastened. However, he added, finding the bodies of the passengers on the crash site with no seats means there was already a state of panic on the plane and the passengers had left their seats.

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It is not clear why Ruvin says people left their seats before the first missile hit. They might have done so between the two impacts since the second missile hit 30 seconds later. There is reason to believe that [something] was burning inside and black smoke was getting out through holes," Ruvin said and added that the team did not find the lower part of the plane and all but four of the seats.

The head of the Ukrainian investigation team also alleged that much of the wreckage of the aircraft and other things found at the scene, including some phones and a tablet, had been removed by the Iranian authorities immediately after the crash.

Read alsoPS752 downing: Ukraine releases intercepted tower communications proving Iran was aware of missile launch all along

Hamed Esmaeiliyon's wife Parissa and daughter Rira were killed in the crash of the Ukrainian plane shot over Tehran in January.

"Maybe someone wrote a text message [about what was happening]," he said and called the Iranian authorities' handling of the situation and the confiscation of all gadgets found at the scene "unprofessional".

Decryption of the flight recorders of the plane may shed light on the situation on the plane before it crashed but so far Iran has held on to the boxes and refuses to hand them over to any third party who can decode the information.

Iran's Revolutionary Guard fired two missiles at Ukraine's flight 752 on January 8 in the wake of Iran's missile attacks on Iraqi military bases hosting U.S forces. Iranian authorities took responsibility for downing the plane and killing 176 passengers and crew members onboard after three days of denials and later claimed that "human error" was responsible for firing missiles.

The interview of the leader of the Ukrainian investigation team has been published amid heightened tensions over the crash between Ukraine and Iran as well as other countries whose citizens were killed in the incident.

Canada, the UK, Sweden, Afghanistan and Ukraine are working together to hold Iran accountable for the crash, Ukrainian Foreign Minister Dmytro Kuleba said in an interview with RBC-Ukraine on May 18.

"The Iranians are very difficult negotiators, but we are not burning bridges and are trying to come to an agreement. But we have red lines," Kuleba said.

“We communicate with each other. Accordingly, if we all fail to reach an agreement with Iran, we will go along a long, but verified and effective way of collective prosecution of Iran wherever possible for the crime that was committed,” he added.

Ukraine's Deputy Minister of Foreign Affairs Yevhen Yenin has said that the countries involved in the matter will be forced to appeal to the U.N. International Court of Justice if negotiations with Iran reach an impasse. "This is not only about Ukraine, but about a united front of the states of Canada, Great Britain, Sweden, Afghanistan and Ukraine," he said.