E.C. I. = exceptionally controlled information, which sounds more like a term coined by a private organization.

NSA als private Gesellschaft? :-)

Siehe den Link in #5

Special Intelligence (SI)

Special Intelligence (so in the CAPCO manual, but always SI in document markings) is the control system covering communications intelligence. Special Intelligence is a term for communications intercepts.[6] The previous title for this control system was COMINT, but this was deprecated in 2011.[7]

• SI has several compartments, of which the following are known or declassified:Very Restricted Knowledge (VRK)
• Exceptionally Controlled Information (ECI), which was used to group compartments for highly sensitive information, but was deprecated as of 2011.[8]

https://en.wikipedia.org/wiki/Sensitive_Compa...

Within the “top secret” classification, there are designations that further restrict access to information, such as “sensitive compartmented information” (the designation of most of the National Security Agency documents released by Edward J. Snowden) and “special access program.”

https://www.nytimes.com/2016/02/05/us/politic...

It appears that Exceptionally Controlled Information is a subcategory of Special Intelligence, which in turn, is a type of Sensitive compartmented information (SCI), which belongs to the top secret classification.