By Brian Todd
(CNN) - Anyone who sends and receives e-mail should be concerned about the investigation that toppled CIA Director David Petraeus.
It's raising serious concerns about privacy, and now we're learning new information about how it unfolded.
E-mails between Jill Kelly and Marine General John Allen were, according to sources, flirtatious. The sources did not provide details, but many of the e-mails may have been innocuous in nature, according to a U.S. official.
But that's still the subject of a Pentagon investigation.
How did it get to that point after starting out as a complaint from Kelley about harassing e-mails from someone else?
The FBI isn't commenting on its investigation, but CNN has new information from Tom Fuentes, a CNN contributor and former FBI assistant director, who spoke to sources with knowledge of the probe.
Fuentes says after Kelley complained to an FBI agent about the harassing e-mails, agents from the Bureau's Cyber Crime Unit got a subpoena and got the internet service provider to give them information on who owned the account of the sender.
That would likely have been more than one internet address. News reports say Paula Broadwell, now known as the person who sent the harassing e-mails to Kelley, is believed to have sent them while she was traveling to various places, so depending on where her computer was plugged in at the time, the sender would have different IP addresses.
Chris Soghoian, an ACLU expert on law enforcement internet monitoring explains what the FBI likely did next.
"What they did is they determined that she'd logged into this e-mail account from hotels around the country and so they contacted the hotels and got the guest lists, the names of the people who were staying in the hotels on those various nights, and then they compared the lists of guests over a few nights at these different hotels and looked for common names, and I guess hers was the only one on that list," said Soghoian.
Fuentes says once they found Broadwell, agents got a search warrant from a U.S. federal judge to read the content of her e-mails.
"During the course of that, they see a flood of other e-mail to an anonymous, from an anonymous person. So they get that person's records. That's where they identify that it's Director Petraeus at the other end of those e-mails with her," said Fuentes.
He says in that process the agents also would have gotten permission to read Jill Kelley's e-mails, and that led them to General Allen.
Soghoian says the FBI likely didn't violate privacy rules.
"Our laws are just out-of-whack with the reality of how we use technology today. The laws haven't been updated, and so the FBI is benefitting from these permissive, out-of-date laws," said Soghoian.
Fuentes adds that the FBI strictly adhered to all privacy guidelines.
He says this investigation was overseen from the start, by top FBI officials in the bureau's Tampa office, and by U.S. attorneys.