The special counsel's report discloses the use of this investigative technique, by which police determine a suspect's location via a cellphone's GPS signal.

The Prince narrative is one instance in unredacted sections of the report in which Mr. Mueller's team explicitly discloses cellphone tracking. It raises the question of whether the FBI applied the process to other investigative subjects — a phone's GPS signal can disclose its exact location within a few feet. One of the first requests the FBI makes when confronting subjects is to ask for their electronic devices.

The fact Mr. Mueller could pinpoint Mr. Prince's exact whereabouts suggest he used GPS readouts, which prosecutors can subpoena from cellular service providers.

"I got the distinct impression that they had all my electronic communications and they operated with a confidence borne of a complete complement of the communications of everyone else," Michael Caputo, a former Trump campaign adviser, told The Washington Times.