A US aircraft attacked a Medecins Sans Frontieres (MSF) clinic in the Afghan city of Kunduz because of "human error", a US military inquiry said.
The investigation found that the crew of the AC-130 gunship mistook the clinic for a nearby government building that had been seized by Taliban.
At least 30 civilians were killed in the 3 October attack, amid a campaign to retake Kunduz from Taliban forces.
MSF said the report demonstrates "gross negligence" by the US military.
The group said the incident constituted "violations of the rules of war" and reiterated calls for an "independent and impartial investigation into the attack".
The US military later admitted that the strike was a mistake and launched an investigation.