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No, it is not illegal, in that there's no law that says that it's a crime. Now if the agent were to compel the CI to trespass, then that could make the agent / agency culpable for a joint crime. It could also result in an exclusion of any evidence. That is, where the CI is himself an "agent" (in an agent / principal relationship, rather than an employee of the agency) and being directed by the principal to do something, then it's as though the principal himself is acting. And if it results in evidence that is procured without "probable cause", then it could be excluded from the trial for that fact. But if it was "allowed" that the CI would do this, but not compelled, then it's not the agency telling or commanding the CI to do this, rather it's the CI's own volition that does it, and would be allowed into court. Likewise, it's not illegal because the agency didn't do anything. It doesn't have an obligation to STOP illegal activity that it knows is going to happen (see the US Supreme Court Castle Rock case).
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable.
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