The 2010 attacks have killed from 143 to 247 people, according to estimates collected by the site, but only seven militants have been publicly identified. Among them are Al Qaeda explosives expert Ghazwan Yemeni, Taliban commander Mohammad Qari Zafar, Egyptian Canadian Al Qaeda leader Sheikh Mansoor, and Jordanian Taliban commander Mahmud Mahdi Zeidan.
Pakistani Taliban leader Hakimullah Mahsud, the architect of a series of suicide bombings and raids on markets, mosques and security installations in the latter half of 2009, was targeted in multiple strikes last year after evidence emerged that he was involved in attacks against the Pakistani government and Americans.
He was initially believed to have been killed in a January drone strike, but apparently survived. This week he appeared in a video, vowing additional attacks against the U.S.
U.S. officials said Wednesday that there is increasing evidence that Mahsud's group, known as Tehrik-e-Taliban, had helped train the Pakistani American who allegedly attempted to carry out a car bombing in New York's Times Square.
The attempt may have been a response to the escalating U.S. drone campaign, one official said.
The number of Predator and Reaper drones in the region is classified, but one former official estimated that the size of the fleet has at least doubled in the last year. The increased numbers improve the CIA's ability to conduct continual surveillance against multiple targets in North Waziristan and other militant strongholds, the officials said.
The CIA maintains a list of senior members of Al Qaeda, the Taliban and other militants, identified by name, whom the agency still tracks and seeks to kill. The decision to widen the program was made because counter-terrorism officials saw militant threats growing, but were unable to use lethal force unless they were able to track a targeted individual.
"In the last year of the Bush administration, the intelligence people had overwhelming evidence that Al Qaeda was regrouping in the tribal areas, and was plotting actively against this country," said the counter-terrorism official.
"You can't hear an alarm like that and then do nothing," the official said, adding that the actions taken by the Bush administration have "intensified since."
The CIA program is operated independently of the U.S. military, which flies its own unmanned aircraft primarily over Afghanistan and follows different targeting procedures.
The border region is a stew of interlocking and shadowy militant groups, some of which seek to attack U.S. troops in Afghanistan or mount larger attacks against U.S. interests, while others are more focused on overthrowing the Pakistani government.
Some outside analysts caution that it could be difficult to determine whether a suspect about whom little is known represents a threat to U.S. interests.
But former officials who were involved in the program said that many of the groups were found to be working together, and thus were considered legitimate targets. One former official directly involved in the program said many locations were watched so closely that the CIA could predict daily routines.
"Is the white van there yet?" the official said, giving an example of the degree of scrutiny. "Is he walking with a limp?"
Officials say some decisions are straightforward -- for example, if drones observe bomb-making or fighters training for possible operations in Afghanistan. In one case cited by officials, a missile was fired at a compound where unknown individuals were seen assembling a car bomb.
People who are determined to be raising money for Al Qaeda or who only facilitate its operations are not targeted, according to a senior administration official. Such support "is not enough as a matter of administration practice and policy to make you a target for lethal operations," the official said.
In addition to more drones, U.S. intelligence agencies involved in the program have increased the number of analysts working on tracking targets and have made other technical upgrades that have improved their ability to track and kill militants.
The Pakistani government occasionally complains publicly about the U.S drone strikes, but also has helped expand the program by providing information about possible targets and by clearing airspace, so the drones can operate without risk of collision with other planes, officials said.