WASHINGTON – On May 6, a CIA drone fired a volley of missiles at a pickup truck carrying nine militants and bomb materials through a desolate stretch of Pakistan near the Afghan border. It killed all the militants – a clean strike with no civilian casualties, extending what is now a yearlong perfect record of avoiding collateral deaths.

Or so goes the U.S. government's version of the attack, from a U.S. 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 six 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-Qaida 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 Barack 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 U.S. 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," 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 U.S. 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, Brennan adjusted the wording of his earlier comment on civilian casualties, saying U.S. officials could not confirm any such deaths.

"Fortunately, for more than a year, due to our discretion and precision, the U.S. government has not found credible evidence of collateral deaths resulting from U.S. counterterrorism operations outside of Afghanistan or Iraq, and we will continue to do our best to keep it that way," he 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. U.S. officials accuse Akbar of working to discredit the drone program at the behest of the Directorate for Inter-Services Intelligence, or ISI, the Pakistani spy service. Akbar and others who know him strongly deny the accusation.

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