The Sept. 11 attacks triggered a revolution in U.S. spycraft as the intelligence services shattered a longstanding taboo by launching an expansive program of targeted killings by remote control.
The intelligence failures that preceded 9/11 rocked U.S. intelligence, prompting an expansion of surveillance in the U.S., and spawning new communications-intercept programs overseas.
But the greatest shift both in tactics and mindset has been the embrace of the pilotless, hunter-killer aircraft known as drones.
Fighting terrorists who acknowledge no boundaries, the CIA has in many ways returned to its World War II roots. Its predecessor, the Office of Strategic Services, was formed in 1942 under William "Wild Bill" Donovan to collect intelligence for the military, organizing guerrilla operations, parachuting into enemy territory and orchestrating sabotage.
"CIA has never looked more like its direct ancestor, the OSS, than it does right now," said former CIA Director Michael Hayden. "It is as intensely operational as it's ever been."
The CIA, which doesn't formally acknowledge the covert program, has killed about 2,000 militants with drones, U.S. officials say, most in the past two years as President Barack Obama's national security team aggressively expanded the program.
In 2010, the number of drone strikes more than doubled, to 114, and this year, drone campaigns are expanding. The CIA now plans flights in Yemen, and the military is using drones to kill militants in Somalia.
"The United States has been fighting al Qaeda for more than a decade now, so it's only logical that counterterrorism would be a top objective for the CIA," said agency spokeswoman Marie Harf. "When the country goes to war, its intelligence agencies do, too. That's always been true, from the days of the OSS in World War II until now."
Legal challenges to the drone program have secured little traction. The main debate inside the government has been over how to execute the campaign without irreversibly damaging Pakistani cooperation.
American citizens can be targets, too. Under the legal authority for the drone program, the CIA must consult the National Security Council before capturing an American posing an imminent threat, but no additional consultation is required to kill an American, a former senior intelligence official said.
"The reason there hasn't been more of an outcry about it is, it's the Obama administration defending this authority," said the American Civil Liberties Union's Jameel Jaffer. "But the authority is going to be used not just by this administration but the next one, and not just the war on terror but the next war."
As the reliance on the drone campaign grows, some intelligence veterans are quietly questioning whether the remote-control killings violate ethical boundaries. "They shouldn't be judge, jury and executioner," said a former U.S. official. "It's an important program, but are there checks and balances?"
American unease with assassination dates back to the CIA's 1960s-era plots to kill figures such as Cuba's Fidel Castro, which spurred President Gerald Ford to issue an executive order banning political assassination.
CIA officials split in the 1980s over how to interpret the ban, according to Robert Chesney, a University of Texas law professor and author of a forthcoming paper on the topic. In 1986, President Ronald Reagan authorized CIA "action teams" to kill terrorists if an attack was imminent, and officials at the time debated what that meant.
The rise of the drone program can be dated to about a decade later, when in 1998 President Bill Clinton authorized the killing of al Qaeda leader Osama bin Laden and his senior associates. But White House and CIA officials disagreed on whether they could kill the terrorist leader.
The next year, with al Qaeda hiding in Afghanistan, a handful of CIA officers looked into using Predator drones to peer into the unreachable territory. The CIA gussied up the Air Force's castoff surveillance Predators and spotted bin Laden in Afghanistan.
"We thought, we need to be able to see him and kill him at the same time," said then-White House counterterrorism adviser Richard Clarke.
The CIA group set out to arm the Predator and had a version in the summer of 2001. But officials decided not to launch it. Some felt that the technology wasn't proven, and others worried that it would be seen as a lethal weapon theCIA didn't have sufficient authority to use..
"We built it, and everyone was getting in a tizzy because it was an 'assassination tool,' " recalled the former U.S. official. Mr. Clarke agreed.
There also were disagreements over who should pay for it, other former officials said. In addition, questions lingered about the craft's missile-firing technology, according to then-CIA Director George Tenet.
After Sept. 11, 2001, President George W. Bush moved to authorize covert action, including for those responsible for the 9/11 attacks. Armed-drone testing grounds quickly moved from the Nevada desert to the mountains of Afghanistan.
In 2002, the CIA and military split drone responsibilities, with the CIA taking Pakistan and the military taking Afghanistan. The U.S. agreed to consult with Pakistan before pulling the trigger, unless it found bin Laden or his No. 2, the former official said.
But by 2006, the U.S. relationship with Pakistan was growing more difficult. The Pakistanis started cutting peace deals with militants in the tribal regions and were slow to respond to drone-hit requests, several American officials said. The U.S. also believed Pakistani intelligence tipped off al Qaeda targets, they said.When there were zero strikes in 2007, then-CIA director Hayden began lobbying Mr. Bush to end the agreement to check with the Pakistanis first. "The frustration with the Pakistanis was the key factor," said one U.S. official. The CIA also wanted to go after militant groups that were targeting U.S. troops on the other side of the border in Afghanistan. After Mr. Bush approved the ramp-up, there were 28 strikes in the second half of 2008.
Mr. Obama had campaigned on refocusing the terrorism fight on al Qaeda. He and his team quickly sought to expand the drone program after his election in 2008. At the same time, the Obama team and worked to reduce civilian casualties. Pakistanis and human rights groups contest CIA claims that civilian deaths have been minimal. On Jan. 1, 2009, the CIA killed two top Al Qaeda planners, reinforcing the importance of the program for the incoming Obama team. Another hit that renewed the administration's commitment, a U.S. official said, was the killing of Pakistani Taliban chief Baitullah Mehsud in August 2009.
That fall, Mr. Obama approved a doubling of the CIA's predator fleet from seven to 14 drone orbits, which usually consist of three planes.