The woman who triggered the investigation that led to the resignation of CIA chief David Petraeus threw lavish parties for top military brass – and also racked up debt. NBC's Kristen Welker reports.

Florida socialite Jill Kelley has emerged as a central figure in the growing scandal that’s ensnared high-ranking officials at the CIA and the Pentagon.

In the latest twist, U.S. General John Allen, commander of the International Security Assistance Force in Afghanistan, is under investigation for exchanging what officials describe as “inappropriate” emails with Kelley, a Tampa, Fla., wife and a mother of three daughters.

That revelation came days after the resignation of Gen. David Petraeus -- a family friend of Kelley’s -- from his post as director of the CIA over an admitted affair.

"This is something that's going to brand her for life," David Khawam, Kelley's brother, told TODAY. "My sister, number one, is a mother. OK, she has three kids. She's extremely dedicated to those kids. Number two, she's a wife. She's extremely dedicated to her husband. And he to her.”

Officials say Kelley’s complaint to the FBI over anonymous, threatening emails touched off the investigation that led to Petraeus’ resignation after officials uncovered an apparent affair with his biographer, Paula Broadwell.

ISAF commander Gen. John Allen under investigation over 'inappropriate' emails

Now, even the FBI agent who first heard Kelley’s complaint is under scrutiny, officials said.

Kelley and her husband, who is a surgeon, are close friends of the Petraeus family. Kelley, 37, was a volunteer social liaison to the MacDill Air Force Base in Tampa, where she often hosted parties for top brass. Kelley also has a twin sister, Natalie, who is a lawyer based in Florida.

According to a senior official, the investigation that led to Petraeus' resignation began several months ago when Kelley reported she had received anonymous harassing emails from a person she didn't know.

The FBI viewed the matter as a potential case of "cyber-harassment" and it was handled "regionally" with federal prosecutors working with the FBI on the matter, the official said. At first, neither Kelley nor the FBI knew who was sending the harassing emails because they came from accounts that were not immediately identifiable.

Petraeus revelation began as cyber-harassment probe; investigation ended 4 days before election

But the FBI was eventually able to determine they came from Broadwell. Investigators then obtained access to Broadwell’s regular email account, where they uncovered her email exchanges with Petraeus and an apparent relationship between the two of them, the official said.

The FBI agent whom Kelley asked to look into the emails she received was a friend and had no further part in the investigation, a senior law enforcement official said Tuesday.

The agent sent Kelley a photo of himself with no shirt, "as a joke, a gag" at least six months before she began receiving the troubling emails, the official said. There is, the official said, no indication that the relationship between the two was anything more than a friendship. Agents discovered the shirtless photo in her e-mails, resulting in an internal investigation into whether he acted inappropriately.

The FBI has declined to identify the agent.

A statement released late on Sunday on behalf of Kelley and her husband, Scott, read: "We and our family have been friends with General Petraeus and his family for over 5 years. We respect his and his family's privacy and want the same for us and our three children."

Frequently fundraising for the Wounded Warriors Project, the Kelleys socialized with Petreaus and his wife, Holly. In 2010, the Petraeuses watched the Gasparilla parade from a tent on the Kelleys front lawn.

Kelley's brother said he stands by her sister and offers his full support.

"The conversations I had with her were basically, that I love you and and we're here for whatever you need," Khawam told TODAY.

In a 1988 Philadelphia Inquirer feature, the Khawams are described as a Lebanese family that emigrated to Philadelphia in the 1970s. Among other activities, they ran a Middle Eastern restaurant called Sahara.

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