Here's a question that I believe is critical to any investigation or possible prosecution.
Who at the State Dept is the authority on what is classified material? Who decides?
Everything I have seen on the subject says the SoS or their appointee has that authority.
Who can classify information?
"In the State Department, original classification authority for top secret info goes to the secretary of state or anyone the secretary has said -- in writing -- can do the job. Past examples include: "Deputy Secretaries, the Under Secretaries, the Counselor, Assistant Secretaries and equivalents; Chiefs of Mission and U.S. representatives to international organizations."
Secret or classified information is decided on by the secretary and/or a senior agency official, who can give classification power to others in writing as well."
Obama on Hillary Clinton’s emails: ‘There’s classified and then there’s classified.’ How does that work?
Since we have no way of knowing what information is now classified, by whom, why and when, then it stands to reason that if it wasn't marked as classified at the time of transmittal then the classifications were within the purview of the Secretary at the time. If Clinton felt the information was not classified then that was her determination to make.
I have read a lot about this and it appears that all of these classifications have come as a result of the review for their public release.
State has said as much and the CIA has even said the classifications weren't necessary.
It seems a prosecution would not be likely.