CIA drone attacks in Pakistan killed at least 581 militants last year, according to independent estimates. The number of those militants noteworthy enough to appear on a U.S. list of most-wanted terrorists: two.
Despite a major escalation in the number of unmanned Predator strikes being carried out under the Obama administration, data from government and independent sources indicate that the number of high-ranking militants being killed as a result has either slipped or barely increased.
Even more generous counts - which indicate that the CIA killed as many as 13 "high-value targets" - suggest that the drone program is hitting senior operatives only a fraction of the time.
After a year in which the CIA carried out a record 118 drone strikes, costing more than $1 million apiece, the results have raised questions about the purpose and parameters of the campaign.
Senior Pakistani officials recently asked the Obama administration to put new restraints on a targeted-killing program that the government in Islamabad has secretly authorized for years.
The CIA is increasingly killing "mere foot soldiers," a senior Pakistani official said, adding that the issue has come up in discussions in Washington involving President Asif Ali Zardari. The official said Pakistan has pressed the Americans "to find better targets, do it more sparingly and be a little less gung-ho."
Experts who track the strikes closely said a program that began with intermittent lethal attacks on al-Qaeda leaders has evolved into a campaign that seems primarily focused on lower-level fighters. Peter Bergen, a director at the New America Foundation, said data on the strikes indicate that 94 percent of those killed are lower-level militants.
"I think it's hard to make the case that the 94 percent cohort threaten the United States in some way," Bergen said. "There's been very little focus on that question from a human rights perspective. Targeted killings are about leaders - it shouldn't be a blanket dispensation."
Even former CIA officials who describe the drone program as essential said they have noted how infrequently they recognized the names of those killed during the barrage of strikes in the past year.
The CIA declined to comment on a program that the agency refuses to acknowledge publicly. But U.S. officials familiar with drone operations said the strikes are hitting important al-Qaeda operatives and are critical to keeping the United States safe.
"This effort has evolved because our intelligence has improved greatly over the years, and we're able to identify not just senior terrorists, but also al-Qaeda foot soldiers who are planning attacks on our homeland and our troops in Afghanistan," said a U.S. official, who spoke on the condition of anonymity to discuss the classified program.
"We would be remiss if we didn't go after people who have American blood on their hands," the official said. "To use a military analogy, if you're only going after the generals, you're likely to be run over by tanks."
The data about the drone strikes provide a blurry picture at best, because of the reliance on Pakistani media reports and anonymous accounts from U.S. government sources. There are also varying terms used to describe high-value targets, with no precise definitions.
Even so, the data suggest that the ratio of senior terrorism suspects being killed is declining at a substantial rate. The New America Foundation recently concluded that 12 "militant leaders" were killed by drone strikes in 2010, compared with 10 in 2008. The number of strikes soared over that period, from 33 to 118.
The National Counterterrorism Center, which tracks terrorist leaders who are captured or killed, counts two suspects on U.S. most-wanted lists who died in drone strikes last year. They are Sheik Saeed al-Masri, al-Qaeda's No. 3, and Ahmed Mohammed Hamed Ali, who was indicted in the 1998 bombings of U.S. embassies in East Africa before serving as al-Qaeda's chief of paramilitary operations in Afghanistan.
According to the NCTC, two senior operatives also were killed in drone strikes in each of the preceding years.
When the Predator was first armed, it was seen as a weapon uniquely suited to hunt the highest of high-value targets, including Osama bin Laden and his deputy, Ayman al-Zawahiri. For years, the program was relatively small in scale, with intermittent strikes.
U.S. officials cite multiple reasons for the change in scope, including a proliferation in the number of drones and CIA informants providing intelligence on potential targets. The unmanned aircraft have not gotten the agency any closer to bin Laden but are regarded as the most important tool for keeping pressure on al-Qaeda's middle and upper ranks.
Officials cite other factors as well, including a shift in CIA targeting procedures, moving beyond the pursuit of specific individuals to militants who meet secret criteria the agency refers to as "pattern of life."
In its early years, the drone campaign was mainly focused on finding and killing militants whose names appeared on a list maintained by the CIA's Counterterrorist Center. But since 2008, the agency has increasingly fired missiles when it sees certain "signatures," such as travel in or out of a known al-Qaeda compound or possession of explosives.
"It's like watching 'The Sopranos': You know what's going on in the Bada Bing," said a former senior U.S. intelligence official, referring to the fictional New Jersey strip club used for Mafia meetings in the HBO television series.
Finally, CIA drone strikes that used to focus almost exclusively on al-Qaeda are increasingly spread across an array of militant groups, including Taliban networks responsible for plots against targets in the United States as well as attacks on troops in Afghanistan.
In recent weeks, the drone campaign has fallen strangely silent. The last reported strike occurred Jan. 23 south of the Pakistani city of Miram Shah, marking the longest pause in the program since vast areas of Pakistan were affected by floods last year. Speculation in that country has centered on the possibility that the CIA is holding fire until a U.S. security contractor accused of fatally shooting two Pakistani men last month is released from a jail in Lahore.
U.S. officials deny that has been a factor and describe the lull as a seasonal slowdown in a program expected to resume its accelerated pace.
The intensity of the strikes has caused an increase in the number of fatalities. The New America Foundation estimates that at least 607 people were killed in 2010, which would mean that a single year has accounted for nearly half of the number of deaths since 2004, when the program began.
Overall, the foundation estimates that 32 of those killed could be considered "militant leaders" of al-Qaeda or the Taliban, or about 2 percent.
The problem does not appear to be one of precision. Even as the number of strikes has soared, civilian casualty counts have dropped. The foundation estimates that the civilian fatality rate plunged from 25 percent in 2004 to 6 percent in 2010. The CIA thinks it has not killed a single civilian in six months.
Defenders of the program emphasize such statistics and say that empirical evidence suggests that the ramped-up targeting of lesser-known militants has helped to keep the United States safe.
The former high-ranking U.S. intelligence official said the drone campaign has degraded not only al-Qaeda's leadership, but also the caliber of the organization's plots.
Thwarted attacks traced back to Pakistan over the past two years - including a botched attempt to blow up a vehicle in New York's Times Square - are strikingly amateurish compared with the attacks of Sept. 11, 2001, and other airline plots that followed, the argument goes.
"Pawns matter," the former official said. "It's always more dramatic to take the bishop, and, if you can find them, the king and queen."
Staff writer Karen DeYoung and staff researcher Julie Tate contributed to this report.