The top US commander in Afghanistan, General John Allen, is under investigation for allegedly inappropriate communication with a woman at the centre of the scandal involving former CIA Director David Petraeus.

The Pentagon says the top American commander in Afghanistan, General John Allen, is under investigation for alleged “inappropriate communications” with Jill Kelley, the woman who is said to have received threatening emails from Paula Broadwell, who had an extramarital affair with former CIA Director David Petraeus.

The shocking revelation threatens to fell another one of the US military’s biggest names and suggests that the scandal involving Petraeus – a former four-star general who had Allen’s job in Afghanistan before moving to the CIA last year – could expand much further than previously imagined.

The US official said the FBI uncovered between 20,000 and 30,000 pages of communication – mostly emails and spanning from 2010 to 2012 – between Allen and Jill Kelley, who has been identified as a long-time friend of the Petraeus family and a Tampa, Florida, volunteer social liaison with military families at MacDill Air Force Base.

It was Kelley’s complaints about harassing emails from Paula Broadwell that prompted an FBI investigation, ultimately alerting authorities to Petraeus’ involvement with Broadwell. Petraeus resigned from his job on Friday.

Benghazi regrets

On Monday, FBI agents searched Broadwell’s North Carolina home, removing bags, boxes and pictures, local media reported. She has not been seen at her home since Petraeus resigned over the affair.

Broadwell may have revealed classified information last month by claiming at a public forum that the CIA was detaining Libyan militia members, a practice which was made illegal by US President Barack Obama in January 2009. Broadwell seemed to imply in her speech that the practice may have triggered the September 11 attack on the US consulate in Benghazi.

“In Washington, this story has very quickly moved on from an extramarital affair to a security issue,” FRANCE 24 correspondent Philip Crowther reported from the capital on Tuesday. US lawmakers are set to hold hearings this week into the Benghazi debacle and claims that the four US victims of the attack were denied sufficient protection because of confusion between CIA and State Department leaders over security responsibility.

Due to the scandal, Petraeus, who was supposed to testify in the affair, will no longer be attending. “The Senate Foreign Relations Committee [charged with holding the hearings] will no doubt be disappointed about that,” Crowther said. “Petraeus did after all go to Benghazi after the attack; he is supposed to be the man who might know the most about why these attacks happened and how they happened.

“Certainly the Benghazi attacks are now slowly but surely becoming issue number one here and not the extramarital affair itself.”

The CIA has strongly denied holding prisoners at their annex in Benghazi.

The scandal has left Obama with a hole to fill on his national security team at a time when he is also expected to be replacing his secretaries of state, defense and treasury.

One name being floated as a possible Petraeus replacement is John Brennan, the White House counter-terrorism adviser and a CIA veteran. Others say Michael Morell, the agency’s acting director, may take on the role.

FBI shenanigans

Petraeus took command of the CIA 14 months ago, retiring from the military after a glittering career that saw him lead the 101st Airborne, the US war in Iraq, its CentCom regional command and international forces in Afghanistan.

Two months after hanging up his uniform, according to retired US Army colonel Steve Boylan, a close Petraeus associate, he started an affair with Broadwell, a former army officer 20 years his junior who traveled with him in Afghanistan and has written a glowing biography of him.

Adding political intrigue to the scandal – according to a report in Tuesday’s Wall Street Journal – the original FBI agent who was contacted by Kelley brought the matter to the attention of Republican lawmakers, who in turn contacted the FBI.

Meanwhile, the FBI investigation itself has come under scrutiny. According to the Wall Street Journal, supervisors pulled the whistleblower FBI agent off the case after he became “obsessed” with the matter and was caught sending Kelley shirtless photos of himself.