Talking to a reporter isn't a crime, and having talked to a reporter isn't enough to get a search warrant. The FBI could have known who the source was, confirm

Talking to a reporter isn't a crime, and having talked to a reporter isn't enough to get a search warrant. The FBI could have known who the source was, confirmed that he would have access to the kind of data being discussed, but couldn't show he had taken classified information so needed to catch him in possession of illegal documents to arrest him.