Remember how someone leaked a presentation on how DEA was using "parallel construction" to prosecute cases when they were initially tipped off of illegal surveillance. They (DEA) might follow the subject and try to weave a chain of evidence that would make seem plausible that an agent accidentally stumbled on the crime somehow (so as to obscure the real source of the information).

Well spy agencies do this very often. For example if a an agent defects (say in this case someone from the Chinese intelligence agency). And they give up the names of 10 agents working for China in US. Well, if FBI goes to their homes immediately and starts arresting them, it will most likely disclose who that original source is. Presumably that kind of information is compartmentalized in China (or any intelligence organization) and once leaked it is trigger a mole hunt on a relatively small group of people (who have been read into that program).

So FBI might start monitoring those spies, and maybe only pick the ones that can do the most damage (or are about to leave the country, for example) and choose to prosecute them. Or I don't know turn them into double agents.