Asha Rangappa is a senior lecturer at the Jackson Institute for Global Affairs at Yale University and a former FBI agent. She is a legal and national security analyst for CNN.

When President Trump fired then-FBI Director James B. Comey in 2017, the bureau opened a counterintelligence investigation into whether Trump was secretly working on behalf of Russia. As a former FBI agent who conducted investigations against foreign intelligence services, I know that the bureau would have had to possess strong evidence that Trump posed a national security threat to meet the threshold for opening such an investigation.

But the more important question now is not how or why the case was opened, but whether it was ever closed. The usual methods for dealing with a national security threat were never intended for a situation in which the president himself is the threat.

The goal of a counterintelligence investigation is to identify and stop threats to national security. Such cases are fundamentally different from criminal investigations, which seek to collect evidence of a crime and are eventually resolved by pursuing or declining to pursue charges in court. By contrast, a counterintelligence investigation is ultimately closed when agents determine that no threat to national security exists or that it has ceased to exist, or when they take actions to render ineffective — in intelligence lingo, to “neutralize” — the threat.

The FBI can neutralize a counterintelligence threat in several ways. One is to simply monitor the activity under the radar and, in the process, collect intelligence on what our foreign adversaries are interested in and able to do. An example of this tactic is the FBI’s Operation Ghost Stories , which allowed 10 Russian spies to believe they were operating undetected for more than 10 years. During this time, the FBI gained important information on Russia’s tradecraft and targets, thereby allowing the United States to enhance its intelligence techniques and foreign policy objectives.

Another avenue to neutralize a target who might be acting on behalf of a foreign power is to cut off that person’s access to information that could help our adversary. In the case of someone sharing classified information with a foreign intelligence agency, for example, the FBI could surreptitiously ensure that the person is no longer able to obtain sensitive information (as it did with Robert Hanssen while he was under investigation in the 1980s and 1990s for spying for the Soviet Union and Russia, giving him a fake “promotion” into a new position in which he didn’t have access to the bureau’s operations on Russian matters), or it could replace the information with fake documents, throwing our adversaries off the trail.

Under certain circumstances, the FBI can take more aggressive steps. If the target is in a position to provide direct intelligence about an adversary and appears to have wavering loyalties, our intelligence services can offer financial and other incentives to “flip” the target — turning him or her into a double agent who provides information to the United States while pretending to cooperate with foreign handlers.

In the case of a foreign national or spy who is working under diplomatic cover and who poses an egregious threat to national security, the FBI can force the target to leave the country — as the Obama administration did when it declared 35 Russian spies persona non grata in December 2016 in retaliation for Russia’s interference in the presidential election.

Unfortunately, none of these are feasible options if the national security threat is the president of the United States.

Naturally, the president, as a U.S. citizen, cannot be removed from the country. Nor can the president, who is the country’s chief executive, be barred from accessing classified information or given falsified documents. It also makes no sense to “flip” someone who is already in a position of public trust and has taken an oath to protect and defend the United States from foreign enemies. Merely monitoring the threat and collecting intelligence on what foreign adversaries are doing is not an option, either, since the ultimate consumer of such intelligence is the president himself — which means whatever intelligence is collected could eventually be passed on to the president, who is also the target. At the same time, the possibility that the president is compromised by a foreign power is the ultimate national security threat: The awesome powers of the presidency, which include almost unfettered discretion in foreign affairs and intelligence operations, leave open the potential for him to use those powers to advance the interests of a foreign adversary over those of the United States.

This leaves only one option for neutralization: exposure.

Exposing the activities of a foreign intelligence service renders them ineffective, since it removes plausible deniability, which is the hallmark of covert intelligence operations. It also reveals the sources and methods that a foreign power is using, forcing the operation to be abandoned. Special counsel Robert S. Mueller III has already utilized this avenue by bringing criminal chargesagainst 13 Russian nationals and three Russian companies for a disinformation campaign on social media aimed at 2016 voters, and against 12 officers of the GRU, Russia’s military intelligence agency, for hacking the Democratic National Committee’s emails .

This alternative has its downsides: It allows our adversaries to know what we know, enabling them to up their game the next time. (The current aggressive attempts by Concord Management and Consulting, which controls Russia’s Internet Research Agency to compel discovery of Mueller’s sources and methods in court is an example of this tension.) But where the national security threat is severe, the need to stop the activity immediately can outweigh the costs.

This is where Mueller’s report comes in. Until now, the American public has seen only snippets of Mueller’s investigation — those he has chosen to make public through criminal charges. But since not all activities uncovered by a counterintelligence investigation, even those that pose a significant threat to national security, are necessarily criminal, the court filings do not reveal the full breadth of what Mueller may have discovered. Only by laying out all of his counterintelligence findings — including what role, if any, Trump played in Russia’s intelligence operation against the United States — can the criminal charges be placed in context and the full scope of the threat be assessed.

If the counterintelligence case against the president was eventually closed because agents found that Trump did not pose a threat to U.S. national security, the president should welcome Mueller’s report. This conclusion would stop the speculation about his relationship with Russia and reassure the American public that his loyalties remain with the United States. But if the case wasn’t closed, and the threat to national security is ongoing, then informing Congress of the nature of the threat is paramount. Then Congress could determine whether it should take the ultimate step to neutralize the damage that the president could inflict on the nation — through impeachment and removal from office.

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