18 U.S. Code ยง 793 -
Gathering, transmitting or losing defense information
(d)Whoever,
>lawfully having possession of, access to, control over, or being entrusted with any document, writing, code book, signal book, sketch, photograph, photographic negative, blueprint, plan, map, model, instrument, appliance, or note
>>relating to the national defense,
>>or information relating to the national defense
>>>which information the possessor has reason to believe
>>>>could be used to the injury of the United States
>>>>or to the advantage of any foreign nation,
>willfully communicates, delivers, transmits or causes to be communicated, delivered, or transmitted or attempts to communicate, deliver, transmit or cause to be communicated, delivered or transmitted the same to any person not entitled to receive it,
>or willfully retains the same and fails to deliver it on demand to the officer or employee of the United States entitled to receive it; or
https://www.law.cornell.edu/uscode/text/18/793
Note that that is not based on the marking of a document.