* Petraeus, Allen intervened in custody battle
* Broadwell's emails showed CIA chief's activities
* Complicates Obama's efforts to revamp national security team
WASHINGTON, Nov 13 (Reuters) - The scandal that felled CIA Director David Petraeus widened on Tuesday to snare the top U.S. commander in Afghanistan, Marine General John Allen, who was being investigated for "flirtatious" communications with a woman at the center of the case.
Though a law enforcement probe has not uncovered any evidence so far of legal wrongdoing or security risks, it does raise questions about the unusually close relationship between a Florida socialite, her sister and two of the most powerful men in the United States' national security apparatus.
The woman in question, Jill Kelley, has emerged as a central figure in the scandal that has brought down one of the most admired military leaders in the United States and threatens to derail the career of another.
Defense officials and people close to Petraeus say neither he nor Allen had a romantic relationship with Kelley, a 37-year-old wife and mother, who is described as a prominent presence in military circles in Tampa.
She may have been seen as a rival by Petraeus' biographer, Paula Broadwell, who sent Kelley a series of anonymous, harassing emails which touched off an investigation that uncovered evidence of an affair between Petraeus and Broadwell.
According to law enforcement sources, FBI investigators decided to pursue the matter when they found the messages contained information about the CIA chief's activities that was not publicly available.
Kelley had gotten to know both Petraeus and Allen as a volunteer setting up social events at MacDill Air Force Base outside Tampa, headquarters of U.S. Central Command.
The relationship was evidently close enough that both men intervened in a child custody battle involving Kelley's twin sister, Natalie Khawam.
"She is a dedicated mother, whose only focus is to provide the necessary support, love, and care for her son," Allen wrote about Khawam in a Sept. 22 letter to a Washington, D.C., court.
Allen and Kelley communicated often enough over the past two years to produce between 20,000 and 30,000 pages of email and other messages, which were turned over to Defense Department investigators on Sunday.
The actual volume of communications is likely much smaller, an official said, as the printouts include messages involving other people and email threads including prior communications.
A senior defense official told Reuters the messages were seen as inappropriate because they were "flirtatious" in nature, not because they dealt with sensitive information.
But "flirtatious" may be an understatement. Another U.S. official said the Pentagon only decided to refer it for investigation after an initial look found the communications to be of "a sufficient character" to warrant further review.
Allen has denied that the two had a sexual relationship, officials said on condition of anonymity. Adultery can lead to a dishonorable discharge under U.S. military law.
WHITE HOUSE BACKS ALLEN
The scandal complicates President Barack Obama's efforts to reorganize his national security team following his re-election. The White House said it still had faith in Allen, but its plans to transfer him to Europe, where he would head U.S. and allied forces, have been suspended.
Obama also has to find a replacement for Petraeus at the CIA at a time when the president is vetting candidates to head the State and Defense departments.
The scandal could throw a wrench into Obama's relations with Congress at a time when he is engaging in high-stakes budget negotiations to avoid the combination of tax hikes and spending cuts known as the "fiscal cliff."
"I certainly wouldn't call it welcome," White House spokesman Jay Carney said of the scandal.
Defense Secretary Leon Panetta said Allen, a four-star Marine Corps general, would stay in his job for the time being, and the White House said Obama was still had confidence in Allen's ability to command the 68,000 U.S. troops in Afghanistan.
Both Allen and the official due to replace him in Afghanistan, Marine Corps General Joseph Dunford, must be confirmed by the U.S. Senate before they can take up their new posts in February.
Lawmakers on the Senate Armed Services Committee said they would go ahead with a confirmation hearing for Dunford on Thursday. Allen's appearance was canceled.
Allen had just submitted recommendations on what role the United States should play in Afghanistan after most American combat troops withdraw by the end of 2014.
A senior lawmaker said on Tuesday the Senate Intelligence Committee still wanted to talk to Petraeus about the CIA's role in events surrounding the September attack on the U.S. consulate in Benghazi, Libya, that killed U.S. ambassador Christopher Stevens and three other Americans.
"The committee will talk to him, the committee thinks it's important," said Senator Dianne Feinstein, the panel's Democratic chairwoman. "The actual time of it is undecided at this point."
FBI agents searched Broadwell's Charlotte, North Carolina, home late on Monday in a sign that the case involving Petraeus was not fully closed.
Agents entered the house carrying boxes at around 9 p.m. (0200 GMT Tuesday) and emerged four hours later, carrying away what appeared to be two computers and about 10 boxes.
Broadwell's family was not at home at the time.
U.S. officials have said recently that their investigation was largely complete and that prosecutors had determined it was unlikely they would bring charges in that case, which started when Kelley contacted an FBI agent in Tampa.
That FBI agent, who has not been identified, came under scrutiny himself after it was discovered he had sent shirtless photographs of himself to Kelley "long before" this investigation, a law enforcement official told Reuters.
The agent, who alerted an FBI cyber squad to the Broadwell case, apparently became frustrated at the pace of the investigation and complained to a member of Congress about it, the official said.