At least one anonymous email sent to Gen. John Allen, commander of U.S. and NATO forces in Afghanistan, was among those turned over by Tampa, Fla., socialite Jill Kelley to the FBI in June, a senior law enforcement official and a source close to Kelley tell NBC News. Kelley’s complaint to an FBI agent with whom she was acquainted triggered the investigation that ultimately led to the resignation of CIA Director David Petraeus.

The law enforcement official, who spoke on condition of anonymity, said Wednesday that Kelley turned over fewer than a dozen emails to the FBI agent, including at least one that Allen had received and forwarded to her.  The emails were ultimately traced to Paula Broadwell, Petraeus’ biographer and the woman with whom he had an extramarital affair, multiple government and law enforcement officials have told NBC News.

The official said it is not clear who received email from Broadwell first -- Kelley or Allen – but a person close to Kelley told NBC News on Wednesday that Allen received the first email in mid-May. According to the source, who also spoke on condition of anonymity, the email sent from an account called "kellypatrol" said , in essence, “Beware of Jill Kelley. She's the kind of person who could ruin you.” It also referred to a meeting Kelley and her husband were planning in Washington with Allen, the source said.

Allen forwarded the e-mail to Kelley, thinking it was a joke from her. She replied that she hadn’t sent it, the source said.

The law enforcement official said Allen also forwarded the email to a “military authority” within the Department of Defense. The official did not specify which office or individual was notified.

Kelley herself began receiving similar emails in early June, the source close to her said, sent from four or five alias accounts. They contained words to the effect of, “What kind of person are you.” And passages directed to her husband, asked, in essence, “Do you know what your wife is up to?” the source said.

As reported previously by NBC News, Kelley took the emails – including at least one from Allen – to the FBI agent she knew because they made reference to meetings she had planned with both Allen and Petraeus, the source said. Kelley wondered why an anonymous e-mailer would know that kind of detail and became concerned that someone was cyberstalking her or hacking into her e-mails, the person said.

Federal officials confirmed Wednesday that the agent who first took the complaint from Kelley is Frederick Humphries, a counter-terrorism agent who worked on the millennium bomb plot case. His name was first revealed by the New York Times.

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Tampa, Fla., socialite Jill Kelley, left, inadvertently triggered the FBI investigation of CIA Director David Petraeus by turning over anonymous threatening emails -- including one sent to Gen. John Allen -- to the FBI.

A senior defense official, also speaking on condition of anonymity, confirmed that Allen had received an anonymous email “some time ago” warning him to be careful around Kelley. The official, who had not seen the email but was generally familiar with its contents, said the general forwarded it to “proper authorities” within the military.

“He did the right thing,” the official said.

The defense official confirmed that the email also was forwarded to Kelley and her husband, Scott, but said it was not certain that Allen was the one who sent it to them. Allen did send Kelley an email referencing the anonymous note he had received, the official said.

The source close to Kelley and several law enforcement officials tell NBC News that Broadwell used multiple anonymous accounts to send the emails to Kelley and Allen. She “covered her tracks” by sending them from cybercafés, the source said.

Slideshow: Petraeus case: Cast of characters

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Meanwhile, defense officials tell NBC News that while there is no evidence that Allen and Jill Kelley engaged in an extramarital affair, there was enough “inappropriate” language in emails they exchanged to warrant an investigation by the Pentagon’s inspector general.

According to one official, who like the others spoke on condition of anonymity, a small number of emails contained language that could be considered “inappropriate” or even “suggestive.” 

But even without those, an investigation of the email correspondence between Allen and Kelley was inevitable given the circumstances, the officials said.

“We had no idea what was going on,” one said. “The last thing we want is to be accused of a cover-up,” regarding the Allen emails. 

Courtney Kube and Mike Brunker of NBC News contributed to this report.

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Then-CIA Director David Petraeus and Gen. John Allen, commander of  U.S. and NATO troops in Afghanistan, intervened in a Washington, D.C., custody battle in September, writing letters on behalf of a woman who was found by a judge to have "severe personal deficits in the areas of honesty and integrity."

The woman, Natalie Khawam, is the twin sister of Tampa socialite Jill Kelley, who has emerged as a central figure in the scandal that led to Petraeus’ resignation last week.

The letters, which have been obtained by NBC News, were filed in court on behalf of Khawam, who the judge hearing the case harshly criticized for a “stunning willingness to say anything, even under oath, to advance her own interests.”

At the time, Khawam was seeking to relax a judge's order restricting her visits with her now 4-year-old son. Holly Petraeus, the wife of the ex-CIA director previously signed an affidavit in support of Khawam, according to the lawyer for Grayson Wolfe, Khawam's ex-husband. The letters were first reported by the New York Post.

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Gen. John Allen, right, commander of U.S. and NATO forces in Afghanistan, and former CIA Director David Petraeus appear ath the White House on April 28, 2011.

The letters came to light after Kelley emerged as a key player in the scandal surrounding emails that officials say were written by Petraeus biographer Paula Broadwell – a trail of correspondence that led to Petraeus’ resignation as CIA director and a Pentagon investigation of Allen over what defense officials have described as “potentially inappropriate” emails he exchanged with Kelley.

The legal battle between Khawam and Wolfe has been bitter, according to court records. Both sides have accused one another of repeatedly lying to the court -- including about invitations to events involving prominent members of Congress.

The court records also shed some light on the lifestyle of the Kelley family: At one point, the judge -- who had directed Kelley's sister to pay child support and the legal fees of her ex-husband -- noted that Khawam lived "rent-free in Florida with her sister" in a home described as a "ten bedroom mansion in a beautiful neighborhood right on Tampa Bay."

The judge also ruled that Jill Kelley was a "patently biased and unbelievable witness" when she testified about an alleged case of domestic abuse by her twin sister's ex-husband.

In a Nov. 9, 2011, ruling, D.C. Superior Court Judge Neal E. Kravitz dismissed Kelley's testimony that she saw Wolfe push Khawam down a flight of stairs inside the Kelleys’ home. Kelley testified that her sister was holding the couple's baby in one hand and "somehow was able to stand her ground on the staircase" as Wolfe, "who is substantially larger and stronger ... pushed Ms. Khawam from above with both hands and all of his might."

Read David Petraeus' letter to the court

Read Gen. John Allen's letter to the court

"The court does not credit this testimony," Kravitz wrote, after calling Kelley an "unbelievable witness." He called it "part of an ever-expanding set of sensational accusations against Mr. Wolfe that are so numerous, so extraordinary and ... so distorted that they defy any common sense view of reality."

Amy Scherzer / Tampa Bay Times via Zuma Press

Natalie Khawam, twin sister of Jill Kelley.

Petraeus and Allen entered the case two months ago, penning separate letters attesting to Khawam’s parenting.

"My wife and I have known Natalie for approximately three years, getting to know her while serving in Tampa, Florida, through her friendship with Dr. and Mrs. Scott Kelley," Petraeus wrote in his letter dated Sept. 20, identifying himself under his signature as "General, U.S. Army (Retired.) "

He added that he has observed Khawam with her son on many occasions, "including when we hosted them and the Kelley family for Christmas dinner this past year." It was "clear to me," he added, that Natalie's son "would benefit from much more time with his Mother and from removal of the burdensome restrictions imposed on her."

Allen's letter, dated Sept. 22, which identified him under his signature as "General, United States Marine Corps," is similar. It stated that he had gotten to know Khawam while serving at the U.S. Central Command in Florida and observed her with her son "on multiple occasions" at "command social functions."

"In light of Natalie's maturity, integrity and steadfast commitment to raising her child, I humbly request your reconsideration of the existing mandated custody settlement," the letter concluded.

Defense official fires back, denies Afghanistan commander exchanged 'inappropriate' emails

A source familiar with Kelley’s views said Tuesday night that both Petraeus and Allen have been friends of Kelley and her sister Khawam for years. The source added:  “When you're involved in a custody issue, you want letters of support. There is nothing unusual about that.” 

Sandra Wilkof, the lawyer for ex-husband Wolfe, said the letters from both high ranking military men misstated the facts of the case. Both letters asked the court to change the terms of a "court settlement" between the couple. In fact, Wilkof, said, "There was no court settlement. There was a court order," she said, awarding custody to Wolfe and supervised visits for Khawam.

Slideshow: Petraeus case: Cast of characters

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Meet the people who have been pulled into the scandal that caused Gen. David Petraeus to resign.

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Judge Kravitz has not given Wolfe a free pass. He wrote in the Nov. 9, 2011 ruling that Wolfe "does not possess an entirely healthy psychological make-up.” And he noted that Wolfe had taken “questionable deductions” on his tax returns and “may have been less than fully candid in his testimony about contacts he may have had with the FriendFinder online dating service.”

But he has saved his harshest words for Khawam, writing that “Mr. Wolfe … is much more honest than Ms. Khawam, and he conducts himself with far greater integrity.”

In the ruling, he found that Khawam had taken the couple's son to Florida when he was only four months old  and refused to tell Wolfe of his whereabouts, ignored court orders to allow visits with his father,  changed the boy's first name without his father's knowledge  and made unfounded claims of abuse against her ex-husband.  

"The evidence established that Ms. Khawam has extreme personal deficits in the areas of  honesty and integrity," Kravitz wrote. "Ms. Khawam's false domestic violence petitions (and her equally false testimony at trial relating to many of the same allegations) are merely the most stunning examples of Ms. Khawam's willingness to say anything, even under oath, to advance her own personal interests at the expense of Mr. Wolfe, the child, and others."

Khawam's lawyer, Greg Jacob, with the law firm of O'Melveny & Myers, did not respond to requests for comment.

In the latest wrinkle in the case, Wolfe's lawyer filed a motion on Oct. 26 opposing the efforts of Khawam to modify the visitation schedule, saying that neither of the letters by Petraeus and Allen promised “corroborating testimony” relevant to the court's determination.

The motion also argued that Khawam had misrepresented social events she had asked the court to let her attend with her son. In one case, Wolfe's motion stated, Khawam had asked the court to let her take her son to a "family clambake" at "the personal invitation" of Rhode Island Sen. Sheldon Whitehouse. In fact, the motion states, this was "nothing more than a … political fundraising event."

On other occasions, the motion states, Khawam had sought to take her son to events with "Senator Kerry" and the baptism of "former Congressman Patrick Kennedy's child" in New Jersey. In fact, the motion states, the invitation to be with Sen. John Kerry was a Democratic Senatorial Campaign Committee event in Martha's Vineyard and that he denied her request to take the boy to those events because he did not believe it was in "the child's best interests."

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