You may make a citizen's arrest, but you would be advised to call the police and let them handle the matter. Police generally have immunity against being sued for arrests; citizens do not. If you happen to be wrong, you could be held liable to the individual in a tort claim called "false imprisonment".
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You would be better off and safer by photographing them and calling the police.
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Mr. Regan and Mr. Garret are correct. Leave the police work to the police. You may collect evidence to assist the police, such as pictures, and if you already know who the individual is you may seek to press criminal charges by talking to the magistrate, but there are severe risks to detaining a person against their will, which is why the advice you have received is to leave the police work to the police.
The police are practically immune to claims of false arrest and kidnapping. You are not. Is it legally permissible to make a citizens arrest? Yes. Should you do it? Probably not. Look at what happened to George Zimmerman in Florida... he was part of a neighborhood watch, saw a young man he did not recognize, and decided to get himself involved in something he should have avoided. Can you take action to try to enforce the law as a citizen? Yes. Can it lead to a physical confrontation? Yes. Can this lead to severe consequences for you (even if you MAY have been in the right?) yes. Can this lead to criminal charges against you and civil charges against you, yes.
As to the fellow in Chantilly, if the facts are as alleged, that a man was peeking into their home at his wife and that the man detained this individual until the police arrived, he is probably ok. This does not mean that he won't be sued, this does not mean that under no circumstances will he face criminal charges. He is probably fine, though.
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Other counsel have all offered excellent advice. But to go back to the basics, have you informed the individual that he is trespassing and is not allowed on the property? Do you have no trespass signs posted?
Also, is there a worn path that others have used for many years to get from one place to another? There may be an issue of prescriptive easement if it's been a regular thoroughfare and no objection was made.
Just some thoughts which may or may not be relevant.
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