WASHINGTON — On May 6, a Central Intelligence Agency 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 United States government’s version of the attack, from an American official briefed on the classified C.I.A. 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 C.I.A.’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, C.I.A. officers believe, the drones have killed more than 600 militants — including at least 20 in a strike reported Wednesday — and not a single noncombatant.