The recent news of a July FBI raid on the home of former Trump campaign manager Paul Manafort has raised expectations in some circles that special counsel Robert Mueller’s investigation is getting closer to issuing indictments against Manafort or others. Certainly, receipt of a warrant to raid Mr. Manafort’s house at the very least suggests that investigators have probable cause that a serious crime was committed.

Based on my experience with espionage investigations however, I suspect that the speed of possible indictments rests on whether the crimes being investigated are financial in nature, or touch upon collusion with Russian intelligence. If the latter, it may be a long time coming before the criminal process wends its way to indictments due to a delicate balance of factors that often arises when a criminal investigation and counterintelligence/counterespionage (CI) investigation overlap. In other contexts at least, the criminal process can also take a back seat to CI interests, allowing known offenders to walk free.

While I believe collusion between the Trump campaign and Moscow is likely (see my New York Times op-ed with Steve Hall), I’m less confident that a CI investigation can lead to satisfying results anytime soon. It is just too hard to prove espionage (broadly defined here to include collusion with a foreign power) in a court of law. Most successful espionage-related convictions result from the target’s ignorance of being in the crosshairs of an investigation and eventually being caught red-handed. Obviously, catching someone in the act in this public and politically charged context is remote. More likely, authorities will look to use separate potential charges in order to convince a co-conspirator to turn and provide evidence on others.

Of course, the focus on Mr. Manafort is not surprising. His years of collecting large payouts from pro-Russian entities suggests that he is willing to work with unsavory characters. Earning money from Russian sources almost always involves at least a brush with corrupt officials or with the Russian security and intelligence services who play a central role in the Russian economy. It is hard to stay clean in the Russian cesspool in which Mr. Manafort has been swimming for a very long time. What’s more, as campaign manager, his participation in the now-famous June meeting with Donald Trump Jr. raises serious concerns. His failure to inform the FBI of the Russian offer of stolen documents was a critical mistake by someone who was undoubtedly aware of Russian intelligence and criminal methods as well as a veteran of major presidential campaigns (it was his fifth).

Recent revelations related to the Trump campaign’s unreported contacts with Russia have also re-surfaced allegations in the leaked dossier from former British intelligence officer Christopher Steele. The Steele report outlines a well-developed conspiracy between the Trump campaign and the Kremlin. According to Mr. Steele’s account, the Russians cultivated Trump associates for years, and were using Manafort, Carter Page and Trump’s personal lawyer, Michael Cohen, as intermediaries. The dossier alleges a flow of Russian intelligence to the Trump campaign (including the supply of derogatory information on Clinton), the use of campaign money to pay Russian anti-Clinton hackers, and conspiratorial meetings between Trump associates and Kremlin operatives. It also describes Mr. Manafort’s receipt of illegal payments from pro-Russian Ukrainian Prime Minister Yanukovych, who fled to Moscow during uprisings in Kiev, and is now wanted for treason by the Ukrainian state.

According to the Steele dossier, Trump associates became ensnared by Russian intelligence as part of the Kremlin’s aggressive targeting of U.S. corporations, visitors to Russia, and banks. The Russians focused on softer targets such as the Trump family after finding it much harder to achieve any success against the harder targets in the U.S. security establishment.

While the accusations in the dossier are impossible for the public to confirm, the report nevertheless provides a wealth of information that investigators with access to intelligence, travel, phone, internet and bank records could use to separate truth from fiction. The report has been panned by some, and even called a possible Russian disinformation effort. If it is a fictional creation of the Russian intelligence services, it certainly is worthy of a Pulitzer for its subtlety, cleverness and complexity. It is important to acknowledge that Steele’s reports, in important respects, anticipated news stories that later turned up similar facts.

Despite the apparent abundance of material to work with, it will be difficult for investigators to develop a strong case of espionage against Mr. Manafort. The complex and strict rules of the criminal justice system do not always mesh with the objectives of a successful counterespionage operation. The primary goal of counterespionage and counterintelligence professionals is to stop the spying before serious damage occurs, while American criminal law is based on a crime already having occurred and proving it beyond any doubt. What’s more, the requirement that national security professionals protect their sources and methods often means that they cannot produce everything they have to prove their case or to explain how exactly they acquired it. This delicate balance often restricts the ability of the government to present the best case possible in any public setting.

A historical example of the constraints on what federal prosecutors can accomplish comes from hunting Soviet spies in the mid to late twentieth century. Hundreds of Americans spied for the Soviet Union during WWII and the Cold War, and only a small handful were ever prosecuted. The government’s methods to uncover spies and its need to protect its sources limited its ability to produce convictions. For example, until its declassification in 1995, the Venona project had decrypted thousands of Soviet intelligence messages uncovering literally hundreds of American spies, including in the White House, OSS and U.S. atomic program. Klaus Fuchs, Alger Hiss, Julius Rosenberg, Harry Dexter White, Duncan Lee, Donald McLean, Maurice Halperin and others were uncovered. However, the sensitive details of the project made it difficult for the government to bring cases to the courts for fear of exposing their code-breaking efforts. A key American physicist and Soviet spy at the Los Alamos atomic laboratory, Theodore Hall, was never prosecuted in order to protect the secret.

Even catching a spy in the act is not a guarantee of a successful conviction. Judith Coplon was apprehended with stolen secret documents in hand while meeting with a Soviet intelligence officer. Nonetheless, the exacting rules of evidence and the government’s need to protect the source of the initial lead, allowed her to walk free.

Over the years, defense attorneys have learned to use the procedures of the justice system to their advantage, demanding access to the government’s sensitive national security information during the discovery process, a tactic so common that it has a name: “graymail.” An aggressive lawyer can scare off the government by insisting on reams of associated secret information as part of the discovery process. For this reason, many potential CI cases are settled out-of-court. There is a natural friction between the legal system’s preference for openness and the imperative of intelligence professionals to protect sources and methods. Once a CI investigation extends into criminal prosecution, the legal discovery process often requires sharing evidence with the defense that intelligence professionals are obligated to protect.

As I’ve written about previously, my primary experience in helping to bring an espionage case to trial involved working closely with the FBI on the 2001 arrest of FBI Special Agent Robert Hanssen. Both CIA and FBI knew that a mole within the national security establishment had been providing secrets to the Russians for some time. Both agencies had been working collaboratively to uncover the spy. Information eventually gathered by the CIA pointed directly to Hanssen as the traitor. However, despite clear indications that Hanssen was guilty, including fingerprints on materials passed to the Russians, and an audio tape of his voice, the Justice Department was reluctant to accept the case. Even though it was clear beyond a shadow of a doubt that Hanssen was a spy with access to tremendously damaging material, they knew that a savvy defense team could interrogate the methods by which Hanssen was uncovered and seek to throw out evidence, especially if a foreign source could not make his/her identity known and testify in open court. As a result, a conviction depended on going a step further and having to catch Hanssen in the act of communicating secret information to the Russians. Luckily, Hanssen did not know that the joint FBI-CIA team was onto him, and he was arrested red-handed loading a dead drop in 2001.

Since the goals, tools and procedures of the CI professional do not lend themselves to swift, if any, justice via the courts, I would suspect that any potential indictment of Manafort or others potentially involved in collusion with the Russians is likely to stem from something other than an espionage charge. The FBI is unlikely to catch Mr. Manafort or any other possible suspects red-handed. If they criminally conspired with Russian intelligence, it was in the past and sufficiently hidden by both sides. Possible evidence will likely come from various intelligence sources, not all of which could be used in court. Further, despite the apparent professionalism of the FBI and Mueller team, any resulting trial could easily descend into a media circus.

So, even if there was collusion between the Trump campaign and Russia, it is unlikely that the government will bring charges of espionage anytime soon. Instead, look for charges on a variety of different issues, and any evidence of collusion to come from leaks or the off-chance that one of the possible conspirators turns state’s evidence. If charges for collusion do not come to pass, it does not mean collusion didn’t occur nor that the intelligence community doesn’t know quite precisely how it happened.