The bloody mayhem allegedly committed by a U.S. soldier in Afghanistan Sunday couldn't have come at a worse time. The killings, which left up to 16 Afghan civilians dead, are likely to inflame an already tense situation fueled by growing Afghan resentment over the presence of U.S. and NATO troops in their country. Recent weeks have seen an upsurge in anti-American protests erupting into violence againstU.S. military and diplomatic personnel.

American officials need to find out the circumstances of the latest killings as quickly as possible and make the results public. They must also hold whoever is responsible fully to account. Anything less risks allowing a single, apparently isolated incident to undermine years of effort to stabilize Afghanistan and prevent it from again becoming a haven for terrorists.

Afghan President Hamid Karzai reportedly was in the midst of a conciliatory speech aimed at persuading his countrymen to accept a U.S.-Afghan security relationship after 2014 when word came that an American soldier had gone on a murderous rampage in southern Kandahar province. The dead included three women and nine children ranging in age from 6 to 9. All the victims had been shot in the head.

Army commanders insisted the killings were unconnected to any military activities in the area and said they had a suspect in custody. The shooter was identified only as a sergeant who recently arrived in Afghanistan after serving three tours in Iraq. Officials said the soldier apparently slipped away from a base during the night and attacked victims in nearby villages before returning and surrendering to authorities, but they offered no motive for the attack.

In the absence of more details about what happened, U.S. officials fear rumors of the incident will ignite more violent anti-American protests across the country and invite a new wave of retaliatory attacks against U.S. and NATO personnel inspired by Taliban propagandists. Reportedly, many Afghans already are skeptical that a rouge soldier acting alone could have carried out so many killings and believe the attacks must have been part of a coordinated U.S. military assault. Some witnesses claim to have seen several people dressed in military uniforms moving through the villages that were attacked.

Such doubts about precisely who and what was involved in the latest killings put new pressure on an already tense relationship between the U.S. and Afghan governments. Complaints among ordinary Afghans that Mr. Karzai isn't doing enough to prevent civilian deaths at the hands of foreign troops are sure to intensify, and the political balancing act the Afghan leader has to perform in response to such criticisms will undoubtedly complicate President Obama's strategy for winding down the war.

The president wants to end the American combat role in Afghanistan by December 2014 and turn responsibility for the country's security over to the Afghan national army and police. American troops remaining in the country after that would be limited to an advisory and training role. But the president's plan depends on getting the Afghan security forces fully up and running well before U.S. troops begin withdrawing. That strategy could be fatally undermined if continuing violent anti-American protests force U.S. troops to leave prematurely.

U.S. officials believed the unrest earlier this month sparked by U.S. troops inadvertently burning copies of the Koran had had begun to die down as a result of extraordinary apologies the president and U.S. Defense Secretary Leon Panetta delivered to President Karzai. Last year, Afghans were outraged by reports that a rogue Army unit that killed several Afghan for sport, then covered up the crimes by claiming the victims were insurgents, and by photographs showing another unit urinating on the corpses of Taliban fighters they had killed. The outrage sparked by all three incidents threatened to derail Mr. Obama's carefully calibrated timetable for withdrawing U.S. troops.

Nor have the Afghans been the only ones impatient for the Americans to leave. A recent poll shows that for the first time a majority of Americans — 55 percent — oppose the U.S. effort in Afghanistan, while nearly as many — 54 percent — favor a military withdrawal even before the Afghan army can stand on its own. Opposition to the war is now almost equally divided among Republicans and Democrats.

Mr. Obama and his military commanders must move quickly to resolve this latest crisis if they are to have any hope of sticking to their timetable for withdrawing from Afghanistan. The fact that a single act of savagery by a lone U.S. service member could threaten the eventual outcome of a decade-long war is an indication of how tenuous the situation there has become — and how easily the best-laid plans for an acceptable outcome there can be overturned. The U.S. needs to get all the facts of this tragedy on the table as soon as possible and then punish those responsible, lest it see all its sacrifice of blood and treasure on behalf of the Afghan people go for naught.