You are working for the CIA as a spy. One day you get a secret offer from the US president to spy on the CIA for him and to go and investigate the truth about a CIA covert operation which happened 10 years ago (let's call it operation X). The US president believes that what the CIA told him about the operation is indeed a lie.
In effect, you have now become a double agent, working for the CIA and secretly for the US president.
After many months of conducting your own secret investigation, you find out that the official CIA report about what really happened during operation X, is in fact a lie. You find the necessary information you need about operation X in various paper only classified documents stored in the CIA document archive.
However, something isn't quite right; getting into the CIA document archive appears to have been much easier that you thought it would.
When presenting the information that you found out during your investigation to the US president, how can you be sure you have the right information? How can the president be sure that you are NOT providing him more false information about operation X?
You could try and confirm the information you have found by secretly interviewing the people involved in operation X, but that would be risky, since they may give you false information and then secretly alert the CIA.