ABC's Alexander Marquardt reports from Moscow: A study by Russian investigators claims to have pinpointed the circumstances around the mysterious 1968 death of Yuri Gagarin, the first man to go into space. The investigation concludes that the first Soviet cosmonaut was flying a Mig-15 fighter jet on a training mission in 1968 when he realized an air vent was open in the cockpit which was supposed to be hermetically sealed. Gagarin quickly put the plane into a dive, plunging the plane from 13,000 feet to 6,500 feet as he had been instructed to do. Plummeting towards the earth, Gagarin and his trainer blacked out and crashed into the forest below. The Telegraph newspaper first reported the theory put forward by former Soviet Air Force Colonel Igor Kuznetsov. He was on the original panel that investigated the crash and in years since has continued to try to unravel the mystery surrounding the flight that killed Gagarin and trainer Vladimir Seryogin. "Nobody knows what really happened except us," Kuznetsov told the paper. "We need to tell our people and the international community the real reason why the world's first cosmonaut died.” The official post-accident investigation was never released, causing a range of conspiracy theories to flourish ranging from the pilots being drunk, to Soviet Leader Leonid Brezhnev ordering Gagarin killed because he was jealous of his fame, to alien abduction. In 2005, the Kremlin turned down a request to open up the archives of the crash. Kuznetsov emphasized to ABC News that in 1968 it wasn’t known that descending too quickly could cause blackouts, he insists that Gagargin and Seryogin followed their training to the letter. Gagarin became a global celebrity in 1961 when he became the first person to go into space and orbit the earth.