Asha Rangappa is an associate dean at Yale Law School. She is a former special agent in the FBI, specializing in counterintelligence investigations. Follow her @AshaRangappa_ . The opinions expressed in this commentary are solely those of the author.

(CNN) For months, we've been hearing the term "collusion" being bandied about -- it's something the Trump administration has repeatedly denied doing with Russia, but an idea that persists the longer special counsel Robert Mueller's investigation continues.

But what does collusion look like, and how would Mueller prove it? Facebook's recent disclosure to Congress of more than 3,000 political ads linked to Russia and purchased during the election can help illustrate how Mueller might move from a counterintelligence investigation into a criminal one, and the steps he would take to make his case for collusion.

Step 1: What did the Russians do?

The key to understanding Mueller's investigation is remembering that first and foremost, it is a counterintelligence investigation . The purpose of a counterintelligence investigation is to stop spies, not find evidence of a crime. Most of the time, these cases never see the inside of a courtroom.

Countries all over the world send their spies to America to obtain defense information, steal trade secrets and push propaganda, among other things. It's the FBI's job -- as part of its counterintelligence mandate -- to detect, monitor, and neutralize the activities of these foreign intelligence services in the United States.

I worked many propaganda -- or "perception management" -- cases for the FBI. The Facebook ads that Russia was promoting would fall into this category, since it was attempting to influence the public's views on the presidential candidates by pushing political ads on controversial issues like LGBT or gun rights on social media.

The key to neutralizing these activities is to make them public and transparent. Mueller has succeeded in doing so by pressuring Facebook to reveal the existence and number of these ads, which it will now also share with Congress.

Step 2: Did they break any laws doing it?

Here's where things get a little trickier for Mueller. Although foreign spies are rarely up to any good in the United States, we have very few laws against "spying." There's something called the Espionage Act, which makes it a crime to share defense information and classified secrets, but Russia's Facebook effort did neither.

In addition, Russian operatives didn't hack into Facebook's system to get its accounts or place its ads -- they did this just like anyone else would, and Facebook's platform allowed them to do it (Facebook itself even approved every ad). It might seem, then, that Russia's disinformation campaign wasn't illegal.

Not so fast. We know that Mueller served Facebook with a criminal search warrant, requesting buyer and ad information on hundreds of accounts. A search warrant can only be issued if a federal judge finds probable cause that a crime may have been committed, and that evidence of that crime is likely to be found in the location searched.

In this instance, federal election law prohibits foreign governments and nationals from making any contributions to an election. If Mueller can trace any of the ads bought on Facebook back to individuals in Russia or the Russian government -- which he likely already has, since a judge signed off on the search warrant in the first place -- he can potentially bring criminal charges against them.

Step 3: Did any Americans help them?

So what does any of this have to do with collusion? Technically, the term "collusion" has no legal significance. However, the behavior that collusion describes -- secretly agreeing to engage in an illegal act -- is a crime; it's called conspiracy.

In order to charge any Americans with conspiracy (or its cousin, "aiding and abetting") with the Russians, Mueller would have to prove that particular individuals knew about Russia's efforts, agreed to help them, and then did something to help the plan along.

At first glance, it might seem that Russia wouldn't have needed any help in its Facebook disinformation campaign. After all, it looks like most of the account holders were located in Russia and operating in secret. But one big clue that they might have received some help from a person or people in the United States is that the ads they placed weren't meant for everyone to see.

Rather, the ads were specifically targeted to specific geographic areas and people with particular ( mostly racist ) preferences. Even a very sophisticated intelligence service like Russia's would have difficulty figuring out this kind of "micro-targeting" on its own.

If any American companies or individuals did have this information and passed it to the Russians with the knowledge that it would be used to create fake Facebook ads, Mueller could charge them with conspiracy or aiding and abetting in violation of federal election law. This would be true even if they didn't know about Russia's "master plan" or all the details of its operation.

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The Facebook ads are not the only aspect of Russia's active measures that could involve a conspiracy. Over the last several months, we've seen evidence that Russian intelligence was conducting secret operations on many fronts, from the hack into the DNC server and subsequent leak of stolen emails to WikiLeaks to the alleged attack on the voting systems in 39 states

If Mueller can prove that Russia was responsible, these actions could constitute crimes like hacking or providing stolen goods. Anyone in the US who assisted them could be charged with conspiracy. Still, the path to this "collusion" is a long and difficult one for Mueller -- but following along in his investigation is, so far, as easy as 1-2-3.