Citing the need to protect the privacy of innocent third parties, a federal judge has restricted the use of cellphone data collected during a criminal investigation. In a written opinion, U.S. Magistrate Judge Iain D. Johnston held that probable cause supports a request for an order allowing the FBI and other law enforcement agencies in the Northern District of Illinois to use a fake cellphone tower to try to get a suspected drug trafficker’s cellphone number. But Johnston also directed authorities to take steps to ensure …