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Frank Taylor's findings challenge the original reports of the Russian and Polish authorities which blamed pilot error for the crash in Smolensk, Russia, in April 2010 which killed 96 people.
In his first television interview, Mr Taylor said: "I think they did not do a thorough investigation. It seems to me they came to that conclusion early on and sought evidence to justify it."
Russian investigators concluded that in dreadful weather conditions the pilot flew too low, hit a birch tree, searing the wing off.
Mr Taylor was involved in the Lockerbie and Manchester air crash investigations. He's examined high resolution photographs of the crash site.
He said: "If the tip (of the wing) had been cut off by a birch tree the damage wouldn't have looked like this. It would have been bent back - consistently backwards, rather than up and maybe even forward.
"As far as I can see there was an explosion in the wing before the aircraft reached the birch tree.
"Possibly some of the bits hit the birch tree afterwards but the evidence is that this explosion caused the wing tip to come off. Then the aircraft rolled but at a higher altitude than the Russian report suggests.
"What I can say is there is no doubt there were explosions on board before the aircraft hit the ground."
Mr Taylor also says he believes damage to the left side cabin door is consistent with what he calls massive internal pressure.
Relatives have long complained that Russia failed to protect the crash site. Russia has refused to return any wreckage or the full flight data recordings.
Political activist Anna Walentynowicz was on the plane. In Poland her son says he was threatened with jail for speaking out after the wrong body was returned to him and that no relative can be sure they have actually buried their loved ones.
Those on board were on their way to the 70th commemoration of the Katyn massacre of Polish intellectuals, politicians and military officers by the Soviets in the Second World War.
A member of the Federation of Katyn Families spoke to us on behalf of the Smolensk relatives but wanted to remain anonymous for security reasons.
He said: "Some relatives have reported being threatened and abused by hooligans. I think it would be fair to describe it as they lived in a climate of fear.
"This is now no longer a Polish problem - this has become a European problem and perhaps an international problem. There now needs to be a cohesive effort to deal with both the air accident investigation side and the human rights violations that have clearly occurred.
"There are some important questions to ask Russia. Most of the relatives are utterly traumatised. They've been waiting for eight years to discover the truth."
Friends and relatives of those who died say an EU resolution calling for an independent inquiry has been ignored.
Even so, they won't give up on their belief that the full story of the Polish air force flight is still to be revealed.
Russia says the crash has already been fully investigated.
source: Sky news