WASHINGTON – CIA Director David Petraeus abruptly resigned Friday after admitting to an extramarital affair, bringing a shocking close to a 37-year career in which he rose to become the Army's leading counterinsurgency strategist, the top U.S. commander in Iraq and Afghanistan and then head of the nation's premier spy agency.

"After being married for over 37 years, I showed extremely poor judgment by engaging in an extramarital affair," Petraeus said in a statement sent to the CIA workforce. "Such behavior is unacceptable, both as a husband and as the leader of an organization such as ours. This afternoon, the president graciously accepted my resignation."

He disclosed no details of the affair, including the identity of the other person involved. But news reports focused attention on his biographer, Paula Broadwell, and indicated the affair came to light when the FBI investigated a possible security breach of Petraeus' email.

Petraeus' departure after only 14 months on the job and three days after President Barack Obama won re-election roiled Washington's national security and political bureaucracies and continued a disruptive trend in which the CIA has seen four leaders depart in eight years. It is the first time in the CIA's 65-year history that the nation's top spy has lost his job over adultery.

Petraeus, who turned 60 on Wednesday, said in his statement, which the CIA made public, that he went to the White House on Thursday to seek Obama's permission to resign.

He didn't comment on the status of his marriage to his wife, Holly, who worked closely with military families while he was on active duty and now handles veterans' financial matters in the Consumer Finance Protection Bureau. They met while Petraeus was a cadet at the U.S. Military Academy at West Point, N.Y., where her father was the academy superintendent.

Broadwell is a research associate at Harvard University, according to an online biography. She spent time observing Petraeus in Afghanistan, while researching her doctoral dissertation on transformational leadership. The resulting biography – "All In: The Education of General David Petraeus" – was released this year.

The precise circumstances that prompted Petraeus to make his adultery public weren't immediately known. But his statement indicated the affair was recent. Keeping it secret could have become a potentially crippling security breach had a foreign power learned of it and used it to try to compromise or blackmail Petraeus.

If he committed adultery while in the Army, Petraeus could have been court-martialed.

Petraeus had been scheduled to testify on Capitol Hill next week as part of the continuing fallout over a Sept. 11 attack on the U.S. Consulate and CIA station in Libya's eastern city of Benghazi. Two of the four Americans killed in the attack were former Navy SEALs on contract to the CIA as security officers. The U.S. ambassador to Libya and another State Department employee also died.

A former aide to Petraeus who has known the general for two decades said he had exchanged emails with him since the scandal broke, and that Petraeus was adamantly against news of his resignation being spun into a conspiracy theory involving the Benghazi tragedy.

"The general insists that he felt this was the right thing to do," said the former aide, who spoke on condition of anonymity because of the sensitivity of the situation. "He insisted that this has nothing to do with Benghazi, nothing to do with Libya, nothing to do with his relationship with the president. Actually, the president took 24 hours to decide on the resignation."

In a statement confirming Petraeus' departure, Obama made no reference to the reason for the resignation. He said that the retired four-star general "has provided extraordinary service to the United States for decades.

"By any measure, he was one of the outstanding general officers of his generation, helping our military adapt to new challenges, and leading our men and women in uniform through a remarkable period of service in Iraq and Afghanistan."

As CIA director, Obama added, Petraeus "has continued to serve with characteristic intellectual rigor, dedication and patriotism. By any measure, through his lifetime of service David Petraeus has made our country safer and stronger."

Obama tapped the agency's deputy director, Michael Morell, a career intelligence officer, as acting director until a replacement for Petraeus is found.

Petraeus wasn't among the top officials who were expected to resign after Obama won re-election Tuesday. His departure will force Obama to devote unexpected energy to finding a new CIA chief as the agency grapples with a host of challenges, from the Iran nuclear crisis and Syrian civil war to drone operations against al-Qaida and allied Islamists in Pakistan and the Middle East.

"I am completely confident that the CIA will continue to thrive and carry out its essential mission, and I have the utmost confidence in Acting Director Michael Morell and the men and women of the CIA who work every day to keep our nation safe," Obama said.

In an unusual statement, Senate Intelligence Committee Chairwoman Dianne Feinstein, a California Democrat, said she wished Obama had rejected Petraeus' resignation request, although she added that she understood and respected the president's decision to accept it.

"At CIA, Director Petraeus gave the agency leadership, stature, prestige and credibility both at home and abroad," Feinstein said. "On a personal level, I found his command of intelligence issues second to none."

Petraeus' steep and sudden fall was devastating for a generation of young officers he personally mentored. The former aide said he had received a stream of emails from other former staff members and veterans of Petraeus' former unit, the 101st Airborne Division, all dismayed but voicing support.

He read excerpts from the messages: "We'd gladly follow him anywhere," one loyalist wrote; another offered, "I'd punch a fool in the face to defend P4," a nickname that referred to Petraeus' four stars.

"My impression is, he's taking a beating, but he's still the same guy I've known for 25 years," the former aide said. "What he's done is horrible, and there's no excuse. But he feels he's doing the right thing by resigning."

Petraeus, from Cornwall-on-Hudson, N.Y., graduated in the top 5 percent of his West Point class in 1974, and married his wife two months later. They have two children.

His Army career centered on assignments in light infantry units, but he also earned graduate degrees at Princeton. He saw combat for the first time in the 2003 U.S. invasion of Iraq, serving as commander of the 101st Airborne Division.

In February 2007, President George W. Bush sent Petraeus back to Iraq as the top U.S. commander to oversee a surge of U.S. forces as part of a strategy to crush the growing Sunni insurgency and prepare for the U.S. troop withdrawal.

In October 2008, Petraeus took charge of the wars in Iraq and Afghanistan as the commander of U.S. Central Command. Obama then sent him to Afghanistan in June 2010 as the top U.S. commander. He developed the effort to train Afghan security forces and oversaw a surge of 33,000 U.S. troops into the Taliban's southern heartland before departing in July 2011 and returning to the United States to retire from the Army and take charge of the CIA.

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