Classified sections of the Pentagon's landmark report on UFO may contain details of alien propulsion tech being probed at Area 51, an investigator has claimed.

US officials released the highly anticipated public document after giving a secret briefing to Congress two weeks ago - but ever since everyone has been asking what is in the full version of the dossier?

Speculation amongst UFO fans has been running rife as lawmakers and officials have all hinted at potentially significant information compiled in the classified section.

Richard Dolan, an American historian who has researched the topic for 25 years, claims one of his sources provided him with details apparently in the section that was briefed to certain US Congressmen and US Senators. 

He alleged the classified portion was 70 pages long and included information about advanced - and potentially alien - propulsion systems and experimental craft using the tech are being tested at Area 51.

However, he made clear the information was "unconfirmed".

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