FBI investigations are long, tedious, and thorough. Even as the latest batch of over 3000 of Hillary Clinton’s emails, released on New Year’s Eve, dribbles out, causing another spike in media controversy on the subject, the FBI continues to methodically pore over every single email to determine if classified information was sent via Clinton’s private server. This investigation might determine the course of the entire 2016 presidential election.

The Clinton campaign has done an excellent job of downplaying the significance of her potential mishandling of classified information, usually conflating it with the relative non-issue of her having a private server that hosted a private email system. But the simple fact Clinton had a private server isn’t the issue for the FBI. The issue is whether or not the server was vulnerable to espionage by a foreign entity or other unlawful access. The FBI doesn’t care if Clinton having a private server broke executive or State Department rules; the FBI isn’t interested if her server was ethical or not.

In fact, the Clinton campaign is entirely correct when it says there’s no criminal investigation into her emails. That’s because the FBI’s investigation is a counter-espionage investigation. Counter-espionage investigations are handled by the FBI’s counterintelligence division, an entirely different FBI unit from the special agents who investigate federal crimes like bank robberies, interstate kidnappings, domestic terrorism, or cybercrime. These agents are spy hunters. Normally, of course, they’re keeping tabs on Russian or Chinese or Iranian spies operating in the United States, looking for the next Robert Hanssen or Aldrich Ames. But they can also be responsible for investigating U.S. vulnerabilities to foreign espionage, such as Clinton’s private server, upon which she conducted government business.

According to a recent Politico article, “Former FBI and Justice officials familiar with the investigative procedures on such matters said the agency must determine two main things: whether the use of an outside email system posed any risks to national security secrets and, if so, whether anyone was responsible for exposing classified information.”