Or so goes the United States government's version of the attack, from an American official briefed on the classified CIA program. Here is another version, from a new report compiled by British and Pakistani journalists: The missiles hit a religious school, an adjoining restaurant and a house, killing 18 people — 12 militants, but also 6 civilians, known locally as Samad, Jamshed, Daraz, Iqbal, Noor Nawaz and Yousaf.

The civilian toll of the CIA's drone campaign, which is widely credited with disrupting Al Qaeda and its allies in Pakistan's tribal area, has been in bitter dispute since the strikes were accelerated in 2008. Accounts of strike after strike from official and unofficial sources are so at odds that they often seem to describe different events.

The debate has intensified since President Obama's top counterterrorism adviser, John O Brennan, clearly referring to the classified drone program, said in June that for almost a year, “there hasn't been a single collateral death because of the exceptional proficiency, precision of the capabilities we've been able to develop.” Other officials say that extraordinary claim still holds: since May 2010, CIA officers believe, the drones have killed more than 600 militants — including at least 20 in a strike reported Wednesday — and not a single noncombatant.

Cutting through the fog of the drone war is important in part because the drone aircraft deployed in Pakistan are the leading edge of a revolution in robotic warfare that has already expanded to Yemen and Somalia, and that military experts expect to sweep the world.

“It's urgent to answer this question, because this technology is so attractive to the US and other governments that it's going to proliferate very rapidly,” said Sarah Holewinski, executive director of the Campaign for Innocent Victims in Conflict, or Civic, a Washington nonprofit that tracks civilian deaths.

The government's assertion of zero collateral deaths meets with deep skepticism from many independent experts. And a new report from the British Bureau of Investigative Journalism, which conducted interviews in Pakistan's tribal area, concluded that at least 45 civilians were killed in 10 strikes during the last year.

Others who question the CIA claim include strong supporters of the drone program like Bill Roggio, editor of The Long War Journal, who closely tracks the strikes.

“The Taliban don't go to a military base to build bombs or do training,” Mr. Roggio said. “There are families and neighbors around. I believe the people conducting the strikes work hard to reduce civilian casualties. They could be 20 percent. They could be 5 percent. But I think the CIA's claim of zero civilian casualties in a year is absurd.”

A closer look at the competing claims, including interviews with American officials and their critics, discloses new details about how the CIA tracks the results of the drone strikes. It also suggests reasons to doubt the precision and certainty of the agency's civilian death count.

In a statement on Tuesday for this article, Mr. Brennan adjusted the wording of his earlier comment on civilian casualties, saying American officials could not confirm any such deaths.

“Fortunately, for more than a year, due to our discretion and precision, the US government has not found credible evidence of collateral deaths resulting from US counterterrorism operations outside of Afghanistan or Iraq, and we will continue to do our best to keep it that way,” Mr. Brennan said.

If there are doubts about the CIA claim, there are also questions about the reliability of critics' reports of noncombatant deaths. Reporters in North Waziristan, where most strikes occur, operate in a dangerous and politically charged environment. Many informants have their own agendas: militants use civilian deaths as a recruiting tool, and Pakistani officials rally public opinion against the drones as a violation of Pakistani sovereignty.

“Waziristan is a black hole of information,” acknowledged Mirza Shahzad Akbar, a Pakistani lawyer who is suing the CIA on behalf of civilians who say they have lost family members in the strikes. American officials accuse Mr. Akbar of working to discredit the drone program at the behest of the Directorate for Inter-Services Intelligence, or ISI, the Pakistani spy service. Mr. Akbar and others who know him strongly deny the accusation.

American officials, who will speak about the classified drone program only on the condition of anonymity, say it has killed more than 2,000 militants and about 50 noncombatants since 2001 — a stunningly low collateral death rate by the standards of traditional airstrikes.

The officials say CIA drone operators view their targets for hours or days beforehand, analyzing what they call a “pattern of life” and distinguishing militants from others. They use software to model the blast area of each proposed strike. Then they watch the strike, see the killed and wounded pulled from the rubble, and track the funerals that follow.

The video is supplemented, officials say, by informants on the ground who sometimes plant homing devices at a compound or a car. The CIA and National Security Agency intercept cellphone calls and e-mails discussing who was killed.

“Because our coverage has improved so much since the beginning of this program, it really defies logic that now we would start missing all these alleged noncombatant casualties,” said an American official familiar with the program.

In one recent strike, the official said, after the drone operator fired a missile at militants in a car and a noncombatant suddenly appeared nearby, the operator was able to divert the missile harmlessly into open territory, hitting the car minutes later when the civilian was gone.

“Nobody is arguing that this weapon is perfect, but it remains the most precise system we've ever had in our arsenal,” the official said.

The agency's critics counter that an intelligence officer watching a video screen thousands of miles away can hardly be certain of the identity of everyone killed in a strike. In a tribal society where men commonly carry weapons and a single family compound can include a militant fighter, an enlistee in the Pakistani government's Frontier Corps, and a shopkeeper, even villagers may be uncertain about the affiliations of their neighbors.

Skeptics likewise say that militants can commandeer a car or a compound from neighbors who cannot safely refuse the demands. And civilians may be present among militants: the Bureau of Investigative Journalism, for example, found that one strike that killed about two dozen militants also killed two civilians, a prisoner of the militants and a visitor negotiating the release of relatives held elsewhere.

The standard drone weapons, Hellfire missiles and 500-pound bombs, like other ordnance, are not absolutely predictable. A strike last October 18, all reports agree, hit a militant compound and killed a number of fighters. But Mr. Akbar, the lawyer, said the family next door to the compound had told his investigators their 10-year-old son, Naeem Ullah, was hit by shrapnel and died an hour after being taken to the hospital in nearby Miram Shah. Neighbors confirmed the account, Mr. Akbar said.

The CIA declines to publicly discuss the drone program, so it was not possible to talk to an agency drone pilot. But Col. David M. Sullivan, an Air Force pilot with extensive experience with both traditional and drone airstrikes from Kosovo to Afghanistan, said remotely piloted craft offered far greater opportunities to study a target and avoid hitting civilians.

An F-117 fighter or a Reaper drone each carries the same 500-pound bombs, “but the Reaper has been sitting for hours on target,” allowing the operator time to study who will be hit by a strike, said Colonel Sullivan, who is on the staff of the secretary of defense.

Still, he said, there is still a margin of error in drone strikes, even if it is far smaller than in traditional strikes.

“Zero innocent civilians having lost their lives does not sound to me like reality,” Colonel Sullivan said. “Never in the history of combat operations has every airborne strike been 100 percent successful.”

American officials said the Bureau of Investigative Journalism report was suspect because it relied in part on information supplied by Mr. Akbar, who publicly named the CIA's undercover Pakistan station chief in December when announcing his legal campaign against the drones. But Mr. Akbar, a former prosecutor, denied he had ever received money or instructions from the ISI, which he said he had often faced off against as a lawyer. He said that in July two ISI agents visited him to ask, “who do you work for?”

Christopher Rogers, an American human rights lawyer who lived in Pakistan in 2009 and 2010, said that he had helped interest Mr. Akbar in the drone strikes and their legal implications. “The idea that ISI was the puppeteer here is not credible at all,” said Mr. Rogers, now at the Open Society Institute in New York.

Though Pakistani officials often denounce the drone program, even as they have at times quietly assisted it, skeptics about its overall impact include American officials as well. The former director of national intelligence, Dennis C. Blair, said at a public forum in Aspen, Colorado last month that he thought unilateral American strikes in Pakistan, Yemen and Somalia should end.

“Pull back on unilateral actions by the United States except in extraordinary circumstances,” said Mr. Blair, who headed national intelligence from January 2009 until May 2010.

C. Christine Fair, an expert on Pakistan at Georgetown University, said that getting full cooperation with Pakistan on drone strikes might be impossible. But Ms. Fair, who said she began as a skeptic but has come to believe that the drones are highly effective and civilian casualties are very low, said the semisecrecy surrounding the program fuels suspicion and allows propaganda to thrive.

The CIA should make public its strikes and their results — even to the point of posting video of the strikes online, she said.

“This is the least indiscriminate, least inhumane tool we have,” Ms. Fair said. “But until there is complete transparency, the public will not believe that.”

Pir Zubair Shah contributed reporting from New York.