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 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.

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.