John Brennan, President Barack Obama's pick to lead the CIA, is a 25-year agency veteran who has passionately defended the US campaign of unmanned drone attacks against Al-Qaeda militants.
Brennan, who has spent the last four years devising counterterrorism strategy in Obama's White House, is an Arabic-speaking Middle East expert who once told reporters when asked about his work ethic: "I don't do down time."
Obama, announcing Brennan's nomination Monday at the White House, said to laughter: "He is one of the hardest-working public servants I've ever seen. I'm not sure he's slept in four years."
Brennan, 57, may have lost out on the top spot at the Central Intelligence Agency in 2009 over his support for the use of certain "enhanced interrogation techniques" under the administration of George W. Bush -- an issue sure to come up at his confirmation hearing.
The CIA veteran's appointment as deputy national security adviser for homeland security and counter-terrorism in 2009 did not require Senate confirmation and thus sidestepped congressional scrutiny.
Brennan has offered the Obama administration's most public defense of drone strikes, the remote-controlled US air attacks on extremist targets that have infuriated many Pakistanis who say that their sovereignty is violated.
In an April speech aimed at increasing openness on drones, Brennan said that the strikes were ethical, proportional and spared innocent civilians from crossfire.
"It is hard to imagine a tool that can better minimize the risk to civilians than remotely piloted aircraft," Brennan said, speaking on US counterterrorism policy one year after the raid that killed Osama bin Laden.
Critics say that drone attacks come with little accountability. A report commissioned by legal lobby group Reprieve in September estimated that between 474 and 881 civilians were among 2,562 to 3,325 people killed by drones in Pakistan between June 2004 and September 2012.
Brennan, in the same speech at the Woodrow Wilson Center for International Scholars, said that bin Laden confessed in documents seized in the raid that Al-Qaeda was losing after "disaster after disaster" inflicted by US forces.
As CIA director, Brennan would replace David Petraeus, who was America's most celebrated military leader in a generation but suddenly fell from grace when he admitted an affair with the co-author of his biography.
Brennan rose quickly within the CIA, serving as a Middle Eastern station chief in 1996 and returning in 1999 to be chief of staff to the agency's longest-serving director, George Tenet.
Brennan witnessed firsthand the struggles between the CIA and the White House over pre-war intelligence on Iraq's weapons of mass destruction and Saddam Hussein regime's links to terrorism.
Brennan then left government for a few years, working in the area of defense and security consulting before accepting Obama's offer to become his counterterrorism czar.
In 2009, liberal bloggers zeroed in on statements he had made in interviews defending the CIA's use of "enhanced interrogation techniques."
He said waterboarding, a form of simulated drowning widely regarded as torture, was inconsistent with American values and should be prohibited.
But in a November 2007 interview with CBS television, Brennan said enhanced interrogation techniques had produced information that the CIA has used against "real hardcore terrorists."
"It has saved lives. And let's not forget, these are hardened terrorists who have been responsible for 9/11, who have shown no remorse at all for the deaths of 3,000 innocents," he said.
In other interviews, Brennan has defended the practice of turning detainees over to foreign intelligence agencies for interrogation, rejecting charges that the program aims to avoid US anti-torture laws.
At the White House, Brennan has been a frequent interlocutor with foreign leaders. He has maintained particularly close contact with officials in Yemen, where Al-Qaeda militants are active.