The American soldier accused of shooting dead 16 Afghan villagers in a pre-dawn rampage has been flown to Kuwait, it has emerged.
The move came as many Afghans called for the unnamed staff sergeant to face justice in their country.
Afghan government officials did not immediately respond to calls for comment on the late-night announcement.
The US military said the transfer did not preclude the possibility of trying the case in Afghanistan and defence secretary Leon Panetta has said the soldier could receive capital punishment if convicted.
The soldier was held by the US military in Kandahar until last night, when he was flown out of Afghanistan to Kuwait, said a US official who spoke anonymously because of the sensitivity of the matter.
Many fear a misstep by the US military in handling the case could ignite a firestorm in Afghanistan that would shatter already tense relations between the two countries.
The alliance appeared near the breaking point last month when the burning of Korans in a rubbish pit at a US base sparked protests and retaliatory attacks that killed more than 30 people, including six US soldiers.
In recent days the two nations made headway towards an agreement governing a long-term American presence, but the massacre in Kandahar province on Sunday has called all such negotiations into question.
Afghan MPs have demanded that the soldier be tried publicly in Afghanistan to show that he was being brought to justice, calling on President Hamid Karzai to suspend all talks with the US until then.
The soldier, who has not been named or charged, is said to have slipped out of his small base in southern Afghanistan before dawn, crept into three houses and shot men, women and children at close range then burned some of the bodies. By sunrise, there were 16 corpses.
The suspect was taken into custody shortly afterwards and at some point taken to Kandahar.
"We do not have appropriate detention facilities in Afghanistan," US Navy captain John Kirby, a Pentagon spokesman, said yesterday. He said that meant a facility for a US service member "in this kind of case".
The US military has detention centres in Kuwait that have been used for other troops. Army PFC Bradley Manning was detained in Kuwait after he was taken into custody in Baghdad in 2010 for allegedly leaking government documents in the WikiLeaks case.
Capt Kirby said the transfer did not necessarily mean the trial would be held outside Afghanistan. US officials had previously said it would be technically possible to hold proceedings in Afghanistan, noting other court-martial trials held there.
But the decision to remove the soldier from the country may complicate the prosecution, said Michael Waddington, an American military defence lawyer who represented a ringleader of the 2010 thrill killings of three Afghan civilians by soldiers from the same Washington state base as the accused staff sergeant.
The prosecutors would not be able to use statements from Afghan witnesses unless the defence is able to cross-examine them, he said.
Mr Waddington said the decision to remove the suspect was probably a security judgment.
"His presence in the country would put himself and other service members in jeopardy," he said.
But the patience of Afghan investigators has already appeared to be wearing thin regarding the shootings in Panjwai district.
The soldier was caught on US surveillance video that showed him walking up to his base, laying down his weapon and raising his arms in surrender, according to an Afghan official who viewed the footage.
The official said there were also two to three hours of video footage covering the time of the attack that Afghan investigators were trying to get from the US military.
US authorities showed their Afghan counterparts the video of the surrender to prove that only one perpetrator was involved in the shootings, the official said.
Some Afghan officials and residents in the villages that were attacked have insisted there was more than one shooter. If the disagreement persists, it could deepen the distrust between the two countries.