Broadwell made many trips to Afghanistan, with unprecedented access to Petraeus, and also spent time with his commanders across the country. When Petraeus took the job at the CIA, she remained in close contact with him, sometimes invited to his office for events like his meeting with Hollywood actress Angelina Jolie.
With the book done, she told friends she had been concentrating on turning part of her research on Petraeus into a dissertation, to complete her doctorate.
Petraeus, in his email, told his CIA employees that he treasured his work with them "and I will always regret the circumstances that brought that work with you to an end."
The director of national intelligence, James Clapper, said Petraeus' departure represented "the loss of one of our nation's most respected public servants. From his long, illustrious Army career to his leadership at the helm of CIA, Dave has redefined what it means to serve and sacrifice for one's country."
Other CIA directors have resigned under unflattering circumstances. CIA Director Jim Woolsey left over the discovery of a KGB mole, and director John Deutch left after the revelation that he had kept classified information on his home computer.
Before Obama brought Petraeus to the CIA, he was credited with salvaging the U.S. war in Iraq.
"His inspirational leadership and his genius were directly responsible — after years of failure — for the success of the surge in Iraq," Sen. John McCain, R-Ariz., said Friday.
President George W. Bush sent Petraeus to Iraq in February 2007, at the peak of sectarian violence, to turn things around as head of U.S. forces. He oversaw an influx of 30,000 U.S. troops and moved troops out of big bases so they could work more closely with Iraqi forces scattered throughout Baghdad.
Petraeus' success was credited with paving the way for the eventual U.S. withdrawal.
After Iraq, Bush made Petraeus commander of U.S. Central Command, overseeing all U.S. military operations in the greater Middle East, including Afghanistan and Pakistan.
When the top U.S. commander in Afghanistan, Gen. Stanley McChrystal, was relieved of duty in June 2010 for comments in a magazine story, Obama asked Petraeus to take over in Kabul and the general quickly agreed.
In the months that followed, Petraeus helped lead the push to add more U.S. troops to that war and dramatically boost the effort to train Afghan soldiers and police.
The Senate and House intelligence committees were briefed on Petraeus' resignation only after the news was reported in the media, said a congressional staffer, speaking anonymously because the staffer was not authorized to publicly discuss the sensitive briefings.
House Homeland Security Chairman Peter King, R-N.Y., said he regretted Petraeus' resignation, calling him "one of America's most outstanding and distinguished military leaders and a true American patriot."
Senate Intelligence Committee Chairman Dianne Feinstein, D-Calif., also regretted the resignation but gave Morell high marks, too.
Morell had served as deputy director since May 2010, after holding a number of top roles, including director for the agency's analytical arm, which helps feed intelligence into the president's daily brief. He also worked as an aide to former CIA Director George Tenet.
"I wish President Obama had not accepted this resignation," Feinstein said of Petraeus, "but I understand and respect the decision."
Associated Press writers Wendy Benjaminson, Ken Thomas, Donna Cassata, Kimberly Dozier, Connie Cass, Eileen Sullivan, Pete Yost and Anne Flaherty contributed to this report.
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