The FBI announced Tuesday that there is evidence Hillary Clinton potentially violated the statutes regarding sending and receiving classified emails when she used private email servers exclusively during her four years as secretary of state, with FBI director James Comey saying her team had been "extremely careless in their handling of highly classified information."

Investigators found 110 emails in 52 chains in Clinton's determined to contain classified email at the time they were sent, in direct contrast to Clinton's message for months that she did not send or receive emails mark classified at the time. But Comey did not recommend the Justice Department press any charges against Clinton, saying investigators did not find "clear evidence Secretary Clinton intended to violate the law."

So what are the government's rules surrounding classified information, and how do they pertain to Clinton? A few months ago, we went through the rule book of how the State Department classifies information. Here's what we found then:

We read the State Department manual on Security Information Regulations and President Obama's official 2010 memorandum on classified information to answer that question. Below are the facts.

What is classified information?

Classified information is simply information that the government deems would be harmful to national security if it were made public. The government's default is that all information is free and open to the public, according to the State Department's manual.

But something can be classified if: "its unauthorized disclosure could reasonably be expected to cause identifiable or describable damage to the national security" AND the information deals with at least one issue bullet below:

(i) Military plans, weapons systems, or operations;

(ii) Foreign government information;**see note below

(iii) Intelligence activities (including covert action), intelligence sources or methods, or cryptology;

(iv) Foreign relations or foreign activities of the United States, including confidential sources;

(v) Scientific, technological, or economic matters relating to the national security;

(vi) United States Government programs for safeguarding nuclear materials or facilities;

(vii) Vulnerabilities or capabilities of systems, installations, infrastructures, projects, plans, or protection services relating to the national security; or

(viii) The development, production, or use of weapons of mass destruction.

Also noted in this section of the code: "In classifying information, the public's interest in access to government information must be balanced against the need to protect national security information."

**NOTE on (ii) Foreign Government Information: This is the kind of classified information Reuters claims to have found on Clinton's email server. It is information that is not the United States' to give  because it's from a foreign government. Examples include something a foreign minister may tell the U.S. secretary of state in confidence, according to J. William Leonard, a former director of the U.S. government's Information Security Oversight Office, who spoke with Reuters. As you'll see below, foreign government information can be tricky to declassify. 

What are the levels of classified information?

Once something meets the criteria to be classified, it goes into one of three categories, laid out by President Obama in his 2010 memorandum. Here they are, from most secret to least secret:

(1) ‘‘Top Secret’’ shall be applied to information, the unauthorized disclosure of which reasonably could be expected to cause exceptionally grave damage to the national security that the original classification authority is able to identify or describe.

(2) ‘‘Secret’’ shall be applied to information, the unauthorized disclosure of which reasonably could be expected to cause serious damage to the national security that the original classification authority is able to identify or describe.

(3) ‘‘Confidential’’ shall be applied to information, the unauthorized disclosure of which reasonably could be expected to cause damage to the national security that the original classification authority is able to identify or describe.

When in doubt, Obama says, classify something at the lower level.

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.

How long does information stay classified?

"For as long as is required by national security considerations," according to the State Department manual, or up to 10 years from the date it was classified.

Some particularly sensitive information can stay classified for up to 25 years.After an item's classification time has expired, it is automatically declassified because, the State Department rules say, no information can stay classified forever.

There are plenty of exceptions. Top officials can intervene to keep something classified, especially when it falls under these categories Obama laid out:

(1) reveal the identity of a confidential human source, a human intelligence source, a relationship with an intelligence or security service of a foreign government or international organization, or a nonhuman intelligence source; or impair the effectiveness of an intelligence method currently in use, available for use, or under development;

(2) reveal information that would assist in the development, production, or use of weapons of mass destruction;

(3) reveal information that would impair U.S. cryptologic systems or activities;

(4) reveal information that would impair the application of state-of-the-art technology within a U.S. weapon system;

(5) reveal formally named or numbered U.S. military war plans that remain in effect, or reveal operational or tactical elements of prior plans that are contained in such active plans;

(6) reveal information, including foreign government information, that would cause serious harm to relations between the United States and a foreign government, or to ongoing diplomatic activities of the United States;

(7) reveal information that would impair the current ability of United States Government officials to protect the President, Vice President, and other protectees for whom protection services, in the interest of the national security, are authorized;

(8) reveal information that would seriously impair current national security emergency preparedness plans or reveal current vulnerabilities of systems, installations, or infrastructures relating to the national security;

(9) violate a statute, treaty, or international agreement that does not permit the automatic or unilateral declassification of information at 25 years.

Is there anything you can't classify?

Yup. There are some rules in place to prevent abuse of the classification system, such as making something secret to avoid embarrassment or conceal breaking the law. From the State Department handbook, here's what you most definitely cannot classify:

(1) In no case shall information be classified in order to conceal violations of law, inefficiency, or administrative error, or to prevent embarrassment to a person, organization, or agency, to restrain competition, or to prevent or delay the release of information that does not require protection in the interest of the national security.

(2) A reference to classified documents that does not directly or indirectly disclose classified information may not be classified or used as a basis for classification.

(3) Only information owned by, produced by or for, or under the control of the U.S. Government may be originally classified.

Can the public request something be declassified?

Yes. So can government officials: In the State Department, an official who believe something is wrongly classified can challenge its status.

A court subpoena is also reason to declassify information before its time. 

The public can file a Freedom of Information Act to see certain classified information. (From the manual: Send your requests for a mandatory declassification review the State Department at: Office of Information Programs and Services, U.S. Department of State, SA-2, 515 22nd St. NW., Washington, DC 20522-8100.)

But the handbook notes that declassification does not necessarily mean something can be released to the public.

The State Department has a few reasons to deny giving the public information, including:

(d) Refusal to confirm or deny existence of information. The Department may refuse to confirm or deny the existence or nonexistence of requested information whenever the fact of existence or nonexistence is itself classified.

(f) Other agency information. When the Department receives a request for information in its possession that was originally classified by another agency, it shall refer the request and the pertinent information to the other agency unless that agency has agreed that the Department may review such information for declassification on behalf of that agency. In any case, the Department is responsible for responding to the requester with regard to any responsive information, including other-agency information, unless a prior arrangement has been made with the originating agency.

(g) Foreign government information. In the case of a request for material containing foreign government information, the Department shall determine whether the information may be declassified and may, if appropriate, consult with the relevant foreign government on that issue. If the Department is not the agency that initially received the foreign government information, it may consult with the original receiving agency.

Foreign government information. There's that term again! It seems that's the one to watch as the FBI looks at all these rules and decides what, if anything, on Hillary Clinton's email server shouldn't have crossed it.