How do we know evidence is not simply destroyed by the FBI/CIA once they are confiscated?

And even if there is unbiased, untampered chain of custody - then how do we know that the files which are generated from said evidence - is also unbiased, untampered with, and unable to be destroyed or altered when billionaires want to keep it secret?

What protect the integrity of the evidence, the files etc? Blockchain? Multiple copies in multiple hands?

A super secret, super trustworthy person who is 100% uncorruptible, and unafraid to release it when given the order?

>>511215115 (OP)
>then how do we know that the files which are generated from said evidence - is also unbiased, untampered with, and unable to be destroyed or altered when billionaires want to keep it secret?

Digital forensics