WASHINGTON - The U.S. staff sergeant suspected of killing 16 Afghan villagers had been drinking alcohol -- a violation of military rules in combat zones -- and suffering from the stress related to his fourth combat tour and tensions with his wife about the deployments on the night of the massacre, a senior U.S. official said on Thursday.

"When it all comes out, it will be a combination of stress, alcohol and domestic issues -- he just snapped," said the official, who has been briefed on the investigation and who spoke on the condition of anonymity because the soldier has not yet been charged.

As new details emerged about possible reasons behind the shootings, the U.S. official said the military was preparing to move the sergeant to a prison in the United States, most likely at Fort Leavenworth, Kan., as early as Friday, just a day after he was flown to a detention site in Kuwait from Afghanistan.

The sergeant's sudden transfer to the United States is the result of a behind-the-scenes diplomatic uproar with Kuwait, which learned of the sergeant's move to a U.S. base on Kuwaiti territory from news reports before the U.S. government could alert the Kuwaitis about it, the senior U.S. official said. "When they learned about it, the Kuwaitis blew a gasket and wanted him out of there," the official said.

The account by the U.S. official, confirmed by a senior official at the Pentagon, is the most detailed description so far of the state of mind of the sergeant, a 38-year-old married father of two who was on his first combat tour in Afghanistan but his fourth overall, including three in Iraq, since he enlisted in 2001.

"There will be questions raised about his emotional and mental stability for a fourth deployment," the U.S. official said.

The Army still has not named the soldier, but on Thursday a lawyer who said he had been retained by his family offered some information and questioned some of the U.S. official's claims.

The lawyer, John Henry Browne of Seattle, said it was "nonsense" that there were exceptional marital tensions.

"I know that is not true," he said at a news conference at his office on Thursday night in Seattle.

Browne added that the inaccuracy of the claim made him "suspicious" of the suggestion that alcohol and stress contributed, although he noted that virtually anyone at a remote base in Afghanistan would be under stress.