The scandal that is now rocking the espionage world traces its roots to e-mail messages.

When investigating the electronic trail, the FBI searched through e-mail messages between the now-former U.S. CIA director David Petraeus and his biographer-turned-mistress Paula Broadwell.

It was revealed this week that the couple often used Gmail to communicate their private, romantic thoughts.

Both they were highly regarded in their professional lives, according to TMCnet and other news sources. Their e-mails were nonetheless used by the FBI to uncover their secrets.

In fact, the entire investigation began when Jill Kelley – a friend of the Petraeus family who has a questionable role in the scandal – first complained to an FBI agent she was getting harassing e-mails which told her to stop socializing with high-ranking U.S. military officers.

As the FBI examined the e-mails, they found they included details about generals in the U.S. Central and Southern Commands, based in Tampa, Fla.

Even though the e-mail accounts were anonymous, FBI agents did see the IP (internet protocol) address of the computers used to send the messages.

Identifying the IP address is the start to any such investigation, Jacques Erasmus, a security investigator at Webroot, told The BBC. "It's definitely one of the first things you do," he told The BBC. "Once you have the IP address you run it through domain tools to get some more information."

The investigators could also use something such as Maxmind to estimate the location of the IP address. "It's roughly accurate," Erasmus told The BBC. "But it won't give you the street and house number."

Broadwell’s name was being seen by investigators repeatedly and the messages were sent from hotels she was staying during her book tour, The BBC said.

“The FBI got a warrant that gave it covert access to the anonymous email account,” The source said. “This led it to uncover evidence of the affair and the ‘trick’ Gen. Petraeus and Ms. Broadwell used to conceal their affair.”

The method let the two of them use the same login and password for the e-mail account. Messages are turned into drafts and reside on a server in a drop-box, where anyone with login information can see them. They are never sent to another account as e-mails.

The FBI also found that the private Gmail account was set up by Petraeus, according to news reports.

It was reported by CNET, too, that Petraeus used e-mail frequently as a communication tool for his job.

The investigation shows how government investigators can use subpoenas, warrants and varied techniques to track down and access e-mail inboxes and information about e-mail accounts with relative ease, The Associated Press reported.

"If the government is investigating a crime, it has a lot of authority to review people's e-mails," Stewart Baker, a former assistant secretary at the Homeland Security Department, told The AP.

"Technology has evolved in a way that makes the content of more communications available to law enforcement without judicial authorization, and at a very low level of suspicion," Greg Nojeim, an attorney at the Center for Democracy & Technology, added in a statement to The AP.

For example, Google (News - Alert), which operates Gmail, complied with some 90 percent of the approximately 12,300 requests it received from the U.S. government last year.

The series of recent incidents raised concern, also, about whether there could have been a security breach involving Petraeus.

Meanwhile, there are many news reports emerging this week that suggests the scandal is widening, and could relate to other issues and incidents.

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