What I mean is: why should we believe them if they say "OK we deleted all copies"? How do we even validate that? I'm very cynical towards intelligence agencies

What I mean is: why should we believe them if they say "OK we deleted all copies"? How do we even validate that? I'm very cynical towards intelligence agencies. They will not let go of what they perceive as valuable intel. Would love to be proven wrong but I don't see how. They can claim anything and everything and do something else where nobody is looking.