That usually means they're trying to piece together which, if any, laws were broken by the employer.

If you don't know the text of the laws and how they apply to circumstances you can't know if they broke the laws. Just employing somebody without authorization isn't illegal--if you check e-Verify and the database is wrong, the government says, "Hey, he can work" and then, if they check and find out he wasn't authorized, it doesn't mean you did anything wrong.

Moreover, each unauthorized worker would be one count, and they can't file a single count until they've confirmed that the worker isn't authorized to work. Imagine that the employer's arrested, charged, and then two weeks later the 15 counts they were initially charged with turn out to rest on 15 people who turned out to be authorized. You have to check these things out--that's how the individuals' cases progress.

And if somebody did something wrong, then they have to determine if the company screwed up or if the company was wronged by its employee. And if the company did something wrong, to what extent can it be shown that they did so intentionally.

Due process vs. vigilantism.