The expanded authority, approved two years ago by the Bush administration and continued by President Obama, permits the agency to rely on what officials describe as "pattern of life" analysis, using evidence collected by surveillance cameras on the unmanned aircraft and from other sources about individuals and locations.
The new rules have transformed the program from a narrow effort aimed at killing top Al Qaeda and Taliban leaders into a large-scale campaign of airstrikes in which few militants are off-limits, as long as they are deemed to pose a threat to the U.S., the officials said.
Instead of just a few dozen attacks per year, CIA-operated unmanned aircraft now carry out multiple missile strikes each week against safe houses, training camps and other hiding places used by militants in the tribal belt bordering Afghanistan.
As a matter of policy, CIA officials refuse to comment on the covert drone program. Those who are willing to discuss it on condition of anonymity refuse to describe in detail the standards of evidence they use for drone strikes, saying only that strict procedures are in place to ensure that militants are being targeted. But officials say their surveillance yields so much detail that they can watch for the routine arrival of particular vehicles or the characteristics of individual people.
"The enemy has lost not just operational leaders and facilitators -- people whose names we know -- but formations of fighters and other terrorists," said a senior U.S. counter-terrorism official, speaking on condition of anonymity. "We might not always have their names, but ... these are people whose actions over time have made it obvious that they are a threat."
In some cases, drones conduct surveillance for days to establish the evidence that justifies firing a missile, the officials said. Even then, a strike can be delayed or canceled if the chance of civilian casualties is too great, they said.
But some analysts said that permitting the CIA to kill individuals whose names are unknown creates a serious risk of killing innocent people. Civilian deaths caused by Western arms is a source of deep anger in both Pakistan and Afghanistan.
"There are a lot of ethical questions here about whether we know who the targets are," said Loch Johnson, an intelligence scholar at the University of Georgia and a former congressional aide. "The danger is that it could spawn new terrorists and increase resentment among the Pakistani public, in particular where these strikes are taking place."
U.S. officials say the strikes have caused fewer than 30 civilian casualties since the drone program was expanded in Pakistan, a claim that is impossible to verify since the remote and lawless tribal belt is usually off-limits to Western reporters. Some estimates of civilian casualties by outside analysts are in the hundreds.
Of more than 500 people who U.S. officials say have been killed since the pace of strikes intensified, the vast majority have been individuals whose names were unknown, or about whom the agency had only fragmentary information. In some cases, the CIA discovered only after an attack that the casualties included a suspected terrorist whom it had been seeking.
The CIA was directed by the Bush administration to begin using armed drones to track Osama bin Laden and other senior Al Qaeda figures, as well as Taliban leaders who fled to Pakistan's tribal areas after the Sept. 11 attacks.
President Bush secretly decided in his last year in office to expand the program. Obama has continued and even streamlined the process, so that CIA Director Leon E. Panetta can sign off on many attacks without notifying the White House beforehand, an official said.
Missile attacks have risen steeply since Obama took office. There were an estimated 53 drone strikes in 2009, up from just over 30 in Bush's last year, according to a website run by the New America Foundation that tracks press reports of attacks in Pakistan. Through early this month, there had been 34 more strikes this year, an average of one every 3 1/2 days, according to the site's figures.
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.