A U.S. Predator unmanned drone armed with a missile stands on the tarmac of Kandahar military airport in Afghanistan. (MASSOUD HOSSAINI/AP)

After a conspicuous lull in the CIA drone campaign in early 2011, the agency’s aircraft roared back to life in mid-March that year, with a multi-missile barrage near a remote village in Pakistan.

The strike at Datta Khel came one day after CIA contractor Raymond Davis was released from a Pakistani jail, and was quickly followed by reports that at least two dozen people — and perhaps as many as 44 — were killed.

Five years later, the true toll of that drone assault remains a mystery. A Taliban commander, Sherabat Khan Wazir, was reportedly among those killed. But the U.S. government has never explained who else died at Datta Khel, whether any of them were civilians, or how it made that determination.

The White House’s long-awaited release Friday of new details about the U.S. drone campaign did little to change that.

People gather at the site of a drone strike on the road between Yafe and Radfan districts of the southern Yemeni province of Lahj August 11, 2013. Local officials and residents in Yemen's southern Lahj Province said a drone destroyed a vehicle travelling on a mountain road late on Saturday evening killing its two occupants and bringing to 15 the death toll from four strikes in three days. (© Stringer . / Reuters/REUTERS)

The newly released statistics help to delineate the overall dimensions of a drone program that grew from an experimental CIA platform designed to hunt Osama bin Laden into a clandestine air force carrying out hundreds of strikes from bases in at least nine countries on two continents.

In making the case for why their casualty estimate of between 64 and 116 civilians killed was the best available — compared with independent organizations with far higher numbers, administration officials said strict rules were followed and outsiders did not have access to the government’s wide-ranging intelligence. That, they said, included everything from overhead surveillance to communications intercepts and human sources before, during and after lethal strikes.

“The purpose here is not to dispute the assessments of conscientious people,” said one of several senior officials who briefed reporters under White House anonymity rules. “It’s simply to be transparent about the information we have based on our vantage point.”

But officials acknowledged that acceptance of what was sharply limited information about what it said were 473 total strikes, conducted by the CIA and the U.S. military’s elite Joint Special Operations over a seven-year period, was largely a matter of trust.

“At the end of the day, this is U.S. government information and people can make their own judgments about how they receive it,” another official said.

Many outsiders found ample room for doubt. Letta Tayler, senior terrorism researcher at Human Rights Watch, led that organization’s efforts to examine seven U.S. strikes in Yemen between 2009 and 2013 that appeared to leave high numbers of civilian casualties. Unlike other estimates based largely on press reports, the group’s research involved visits to the sites of strikes and extensive interviews with witnesses, relatives and Yemeni officials. Overall, the organization concluded that at least 57 civilians, and possibly as many as 59, were killed in those seven strikes.

This video showing Warren Weinstein, a U.S. contractor held by al-Qaeda militants, was released in 2013. The full 13-minute video was sent anonymously by e-mail to several journalists who have reported from Afghanistan. (As-Sahab/AFP)

“I find it difficult to believe that in examining just seven attacks I happened upon well over half of the civilian deaths that the U.S. acknowledges,” Tayler said.

The strike that reportedly produced the highest number of civilian casualties came toward the end of Obama’s first year in office, when ship-launched cruise missiles carrying cluster munitions killed 14 suspected al-Qaeda fighters in Yemen but also as many as 41 civilians. According to Human Rights Watch and an investigation by the Yemeni government, there were 21 children and nine women among them.

The fallout from that attack put pressure on the White House to adopt stricter targeting guidelines and abandon the use of cruise missiles in counterterrorism operations. By 2013, Obama had imposed rules requiring evidence of an imminent threat to the United States and “near certainty” that no civilians would be killed.

But a JSOC strike in December, 2013, on a train of vehicles leaving a wedding in Yemen exposed how even inside the U.S. government there could be intense disagreement over how to distinguish combatants from civilians.

The so-called “wedding strike” killed 12 people. JSOC concluded that all 12 were militants, according to U.S. officials briefed on the operation. But CIA analysts cast doubt on that claim, saying they could identify only one or two militants. The White House turned to the National Counterterrorism Center to referee the dispute, and its assessment concluded that as many as half of those killed were civilians.

This week’s release left unclear whether any of those killed in that operation were counted as civilians in the seven-year total.

Even before the data release, the Obama administration has frequently depicted the drone campaign as almost unerringly precise. In 2011, near the peak of the drone war in Pakistan, Obama’s top counterterrorism adviser John O. Brennan — who now serves as CIA director — said that for nearly 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.”

But those assertions have been undermined by the administration’s acknowledgment of grievous errors in recent years, including several that unintentionally killed U.S. citizens. Of the eight Americans killed in U.S. strikes, only one — Anwar al-Awlaki, a U.S.-born cleric who became a senior al-Qaeda operative in Yemen — was deliberately targeted.

One American, aid worker Warren Weinstein, was killed in a CIA drone strike on an al-Qaeda compound in Pakistan last year. The agency was unaware that Weinstein and an Italian citizen, Giovanni Lo Porto, both kidnapped by al-Qaeda, were being held at the compound it targeted.

Former U.S. officials involved in overseeing the drone program said that at times even they were unsure about how the agency and JSOC categorized casualties. The confusion only deepened, they said, as the administration ramped up its use of “signature strikes” in which targets are selected based on patterns of behavior seen as denoting terrorist activity even when the identities of those who would be killed is unknown.

As the drone war heated up in Obama’s first term, signature strikes came to account for more than two-thirds of the shots taken in Pakistan. A former senior U.S. official involved in overseeing the campaign said that in one briefing CIA officials argued that no civilians were left in the areas being patrolled by drones.

“Everybody who is in those zones is a combatant because everybody who is not a combatant has left,” the former official said. Like others, he spoke on condition of anonymity to discuss sensitive counterterrorism operations. The official said he and others pushed Obama to release more information on the drone program, but that intelligence agencies fought the proposals largely because they feared being drawn into a public fight over statistics.

“The main concern about putting out numbers was that people would ask for methodology — what is your test for distinguishing combatant and non-combatant?” the former official said. “The intelligence community] always said, ‘We’ll never get praise for [releasing drone data] so why do it?’”

Karen DeYoung and Julie Tate contributed to this report.