The US Justice Department has decided not to charge former CIA director David Petraeus' biographer and former mistress, Paula Broadwell, with cyberstalking as part of its investigation into an email scandal that led to the storied general's resignation.

Broadwell's lawyer, Robert Muse, gave The Associated Press a letter from US Attorney Robert O'Neill that said no federal charges will be brought related to "alleged acts of cyberstalking."

Petraeus resigned in November after acknowledging the extramarital affair, which was exposed after Broadwell emailed Florida socialite Jill Kelley, allegedly warning Kelley to stay away from Petraeus and General John Allen, the US commander in Afghanistan.

Kelley reported the emails to the FBI, triggering an investigation that also led the FBI to Kelley's emails to the married Allen, who is now under investigation by the Pentagon's inspector general.

"The decision on whether to bring a prosecution is always a serious matter, and one that should never be undertaken without the most thoughtful deliberation," said Justice Department spokesman William Daniels.

A spokesman for Broadwell said she and her family are "pleased with this decision and pleased that this is resolved."

Her attorney has not been notified that she is the subject or target of any other Justice Department investigation.

Broadwell is still being investigated by the Defense Department for allegedly mishandling classified information. FBI investigators found a "substantial amount" of material marked classified at her home.

The documents were part of her research from her trips to interview Petraeus and his commanders across Afghanistan for her best-selling book on Petraeus, All In, published earlier this year.

Petraeus told friends that he had never given classified information to Broadwell, and she said she didn't receive such material from Petraeus.

Petreaus and Broadwell say their romantic relationship began only after he retired from the military and started at the CIA.

Both Petraeus, 60, and Broadwell, 40, are married. Broadwell has two young children.

Petraeus and Broadwell have both expressed regret for the affair, which they say ended this summer.

The CIA is investigating Petraeus' conduct to examine whether he may have used CIA resources to further the affair, but the Pentagon has shown no appetite for recalling Petraeus to active duty in order to punish him for adultery, which is illegal under military law.

Kelley befriended top military brass as part of her volunteer work hosting elaborate parties for Petraeus and other top commanders at Central Command, a huge military base in Tampa.

That role eventually won her role a post as an "honorary consul" to South Korea. She has since been stripped of that post.

- AP