Hillary Clinton in Durham, N.H., on Thursday. (Matt Rourke/Associated Press)

The Hillary Clinton email issue is developing into a real whodunit, complete with Clintonesque legal semantics. “I never sent or received any material marked classified,” she said with respect to the discovery of classified information on her private, unclassified email server. That surface denial nearly rivals Bill Clinton’s classic: “I did not have sexual relations with that woman.”

But this is no laughing matter.

There is nothing trivial about a secretary of state having top-secret information on an unsecured computer in her home. That appears to have been the case, based on the State Department’s announcement last week that 22 emails, across seven email chains, containing top-secret information were on Hillary Clinton’s private email server.

At issue is whether the information in the emails was classified when it was sent to her unsecured server. It was, after all, the State Department, upon review of the content by intelligence agencies, that upgraded the emails to top-secret and ordered them withheld from the public.

Now, it may well be that some of Clinton’s political opponents are out to derail her presidential campaign and are using the email controversy to do so. Or it could be the case, as Clinton’s supporters claim, that intergovernmental infighting over what is and isn’t classified is driving this investigation.

CNN host Chris Cuomo questioned Hillary Clinton on the email server she used during her time as Secretary of State during the Jan. 25 town hall in Des Moines, Iowa. (CNN)

The important nonpolitical question: Did the nation’s top diplomat or her State Department staff improperly handle extremely sensitive, top-secret information and do so in a manner in which the information could be compromised?

State Department rules are quite clear.

Top-secret information must not be placed on any unclassified systems. It must be accounted for and controlled. And no copy of a top-secret document can be made without the permission of the office or agency in which it originated.

In addition, any State Department employee who causes the compromise of top-secret information or makes a copy of a top-secret document or any portion of it without the originator’s permission is subject to administrative action.

There are also limited ways in which top-secret information can be transmitted. Sending top-secret information via a private, unsecured email server is not one of them. Transmitting top-secret information with the classification removed is also forbidden.

That makes it critical to establish whether Clinton’s private server contained information that was classified at the time it was sent or received.

Clinton campaign spokesman Brian Fallon said , “She was at worst a passive recipient of unwitting information that subsequently became deemed as classified.” In other words, Clinton is an innocent victim of bureaucratic infighting. If so, how did it happen?

That’s what makes this a Washington whodunit.

Someone inside the State Department transmitted the information to Clinton’s personal email account through a private server. That employee — or employees, as the case may be — knows or should know whether the material was drawn from, was based on or included top-secret information.

Given that the information on the server has been upgraded to top-secret, another fear arises: Have unauthorized individuals, even foreign governments, gained access to highly classified information, to the detriment of the United States?

It’s not as though clandestine attempts to penetrate government agencies have not been made.

In fall 2014, the State Department shut down and shored up its unclassified email system after detecting a possible hacker attack. A hacker also attacked the White House’s unclassified computer system around the same time.

Last year, Iranian hackers broke into the email and social media accounts of State Department officials who focused on Iran and the Middle East, according to the New York Times.

In July, The Post reported that hackers who attacked the Office of Personnel Management got the personnel and security files of at least 22 million people, including federal employees and contractors, as well as their families and friends.

The U.S. Postal Service was hacked in 2014. The National Oceanic and Atmospheric Administration’s computer system was compromised the same year.

The nongovernment personal accounts of CIA Director John Brennan and Homeland Security Secretary Jeh Johnson reportedly have been hacked.

It’s chilling to think of what a breach of Clinton’s email account might mean to national security.

Presidential election year or not, the Clinton email issue must be resolved. Just a thought: As a precaution, the manager in the White House dugout might consider telling the bullpen to start warming up Joe Biden.

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