Why are you asking me questions that are already answered in our law? Due process and the limits on our government in investigating crimes are laid out in the Constitution, and examination of other laws clarifying these things should answer your question.
To point out that privacy has not been generally considered to be a right and unlike restricting government from regulating speech or RTBA, it doesn't make much sense to restrict government from invading privacy as privacy is commonly defined and traditionally understood and note: property and freedom of movement are different than privacy.