If the condominium is being used for criminal purposes, it can be seized by law enforcment agencies. If you want to buy this condominium, you should consult an experienced real estate lawyer in your area and make sure that you get title insurance.
Disclaimer: This answer is provided for informational purposes only, does not constitute legal advice, and does not create an attorney-client relationship. Actual legal advice can only be provided after completing a comprehensive consultation in which all of the relevant facts are discussed and reviewed.
Depending upon what he's being investigated for, you might want a home inspection (meth lab?) and/or worry about your safety from those who may not like him, and not know when he's no longer there...
My advice, of course, is general and based on very little knowledge of your personal situation, and I am licensed to practice law only in Idaho, Utah and Wyoming. You should retain legal counsel and review you situation with him or her, if it has significant financial or other implications.
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