WASHINGTON -- The FBI started its case in June with a collection of five emails, a few hundred kilobytes of data at most.
By the time the probe exploded into public view earlier this month, the FBI was sitting on a mountain of data containing the private communications -- and intimate secrets -- of a CIA director and a U.S. war commander. What the bureau didn't have -- and apparently still doesn't -- is evidence of a crime.
How that happened and what it means for privacy and national security are questions that have induced shudders in Washington and a queasy new understanding of the FBI's comprehensive access to the digital trails left by even top officials.
FBI and Justice Department officials have vigorously defended their handling of the case. "What we did was conduct the investigation the way we normally conduct a criminal investigation," Attorney General Eric Holder said Thursday. "We follow the facts."
But in this case, the trail cut across a seemingly vast territory with no clear indication of the boundaries, if any, that the FBI imposed on itself. The thrust of the investigation changed direction repeatedly and expanded dramatically in scope.
A criminal inquiry into email harassment morphed into a national security probe of whether CIA director David Petraeus and the secrets he guarded were at risk. After uncovering an extramarital affair, investigators shifted to the question of whether Mr. Petraeus was guilty of a security breach.
When none of those paths bore results, investigators settled on the single target they are scrutinizing now: Paula Broadwell, the retired general's biographer and mistress, and what she was doing with a cache of classified but apparently inconsequential files.
The investigation's profile has called attention to what legal and privacy experts say are the difficulties of applying constraints meant for gathering physical evidence to online detective work.
Law enforcement officers conducting a legal search have always been able to pursue evidence of other crimes sitting in "plain view." Investigators with a warrant to search a house for drugs can seize evidence of another crime, such as bomb-making. But the warrant does not allow them to barge into the house next door.
But what are the comparable boundaries online? Does a warrant to search an email account expose the communications of anyone who exchanged messages with the target?
Similarly, FBI agents monitoring wiretaps have always been obligated to put down their headphones when the conversation is clearly not about a criminal enterprise. It's known as minimization, a process followed by intelligence and law enforcement agencies to protect the privacy of innocent people.
"It's harder to do with emails, because -- unlike a phone -- you can't just turn it off once you figure out the conversation didn't relate to what you're investigating," said Michael DuBose, a former chief of the Justice Department's Computer Crime and Intellectual Property Section who now handles cyberinvestigations for Kroll Advisory Solutions.
It's unclear whether the FBI made any attempt to minimize its intrusion into the emails exchanged by Ms. Broadwell and Mr. Petraeus, both of whom are married, that provided a gaping view into their adulterous relationship.
Many details surrounding the case remain unclear. The FBI declined to respond to questions submitted by The Washington Post on its handling of personal information in the course of the investigation. The bureau also declined to discuss the broad guidelines for safeguarding the privacy of ordinary citizens whose emails might surface in a similarly inadvertent fashion.
Although the Petraeus-Broadwell investigation ensnared high-ranking officials and had potential national security implications, the way the FBI assembled evidence in the case was not extraordinary, according to several experts.
The probe was triggered when Jill Kelley, a Florida socialite with ties to Mr. Petraeus and Gen. John Allen, the U.S. military commander in Afghanistan, went to the FBI in June with menacing emails from an anonymous sender. The emails were eventually traced to Ms. Broadwell, who thought Ms. Kelley was a threat to her relationship with Mr. Petraeus, officials said.