The military scandal gripping America took an extraordinary twist yesterday, when a second general, John R. Allen, was linked to one of the two women who precipitated the fall of CIA director General David Petraeus.

David Petraeus, 60, hailed by some as the finest officer of his generation, was forced to step down after the exposure of a scandal involving X-rated emails, compromised state secrets and an affair forged in the ruins of Afghanistan.

His mistress was revealed as Paula Broadwell, a glamorous defence academic and ex-army officer, who had written a fawning biography of David Petraeus after she was “embedded” with him in Afghanistan.

Though they conducted their affair via private email accounts and with sufficient discretion to hoodwink even the suspicious minds of his CIA subordinates, it came to light thanks to old-fashioned female jealousy.

Paula Broadwell became convinced that she was, in fact, involved in a love triangle – and not one that involved David Petraeus’ wife, Holly.

The “other woman”, a 37-year-old Florida socialite and mother of three named Jill Kelley, went to the FBI after receiving threatening emails from Paula Broadwell.

But, almost incredibly, the investigation she set in train not only led to the demise of David Petraeus, but has ensnared another senior figure, General John Allen.

Yesterday, the Pentagon announced that General John Allen, 58, – who was about to be appointed NATO’s Supreme Allied Commander in Europe – is being investigated for “inappropriate communication” with Jill Kelley.

Paula Broadwell, a glamorous defence academic and ex-army officer, had written a fawning biography of David Petraeus after she was embedded with him in Afghanistan

While investigating Jill Kelley’s case, the FBI discovered what it says amounts to between 20,000 and 30,000 pages of emails between her and General John Allen between 2010 and 2012. Yesterday, some of those emails were described by a senior defence official as “flirtatious”.

John Allen succeeded Petraeus as the head of U.S. forces in Afghanistan, but his new European post is now on hold.

Like David Petraeus, John Allen got to know Jill Kelley when he was stationed at Central Command near her home in Tampa, Florida, where she organizes social events for senior military families.

Not only has the widening scandal been disastrous for the Petraeus, Broadwell and Kelley families, but it has also caused major ructions in Washington as the first nightmare of the new Barack Obama presidency.

Some FBI agents considered the missives too trivial to warrant following up, but – encouraged by Jill Kelley’s FBI chum – investigators spent weeks tracing where the emails had been sent from, and eventually matched them to hotels where Paula Broadwell had been staying.

Gaining a warrant to monitor her emails, they discovered Paula Broadwell and David Petraeus – using a pseudonym – had set up private Google Mail accounts to send steamy messages to each other.

(Terrorists use the technique, by which the sender can leave messages in a “draft” folder so the recipient can log in and read them without anything being emailed.)

By late summer, the FBI had worked out the general was involved and – alerting the U.S. Justice Department and Attorney General Eric Holder – they interviewed Paula Broadwell in September.

Paula Broadwell admitted the affair and agreed to hand over her computer. On its hard drive, investigators found copies of undisclosed classified documents – a discovery that inevitably rang even louder alarm bells. (Federal agents returned to her home on Monday night and removed boxes of documents and two computers.)

Confronted by the FBI with its evidence the week before the presidential election, David Petraeus also admitted to the affair, but insisted he hadn’t sent the secret material to his mistress.

Officials are reportedly convinced David Petraeus is telling the truth, but still don’t know how the documents were found on Paula Broadwell’s computer.