There must be probable cause to believe that a crime has been committed and that the target of the investigation was the one who participated in committing it. It must be more than a suspician based on the fact that someone is a nefareous character.
The investigation must find at least a sufficient basis for obtaining a warrant.
In this case, I believe that the fact of the warrant was not the issue, it was that there is a state statute that specifically required notification of the target of the investigastion when it related to medical records, in particular, so that the target of the investigation could apply to the court before his confidential medical records were reviewed.