(Reuters) - The soldier implicated in the massacre of 16 villagers in Afghanistan this week, an incident that sent American-Afghan relations into a tailspin, is U.S. Army Staff Sergeant Robert Bales, a U.S. official said on Friday.

The official declined to provide additional details about Bales, who is suspected of walking off his base in southern Afghanistan on Sunday and gunning down the 16 villagers, including nine children and three women.

Earlier, the soldier's attorney said the staff sergeant was scheduled to arrive on Friday at the Fort Leavenworth army base in Kansas, where he will be held in maximum security. Fort Leavenworth has the U.S. Defense Department's only maximum-security facility.

The 38-year-old soldier, whose military unit is based south of Tacoma, Washington, had been held in Kuwait after he was flown out of Afghanistan on Wednesday.

Bales has not yet been charged in the incident. U.S. military officials have said they would not publicly identify the soldier until he is charged.

I would assume he'll be charged pretty fast, said Jeffrey Lustick, a defense attorney and former Air Force military prosecutor and defense attorney in Bellingham, Washington.

The soldier is expected to face justice under U.S. military rules, but it is not clear where any trial would take place.

Photos of a soldier identified as Bales, wearing camouflage and battle gear, appeared in an article about training for soldiers headed for Afghanistan on a web publication linked to Fort Irwin, a California military base. According to the website, the photos were taken in August.

Bales' wife and two young children have been moved to Joint Base Lewis-McChord near Seattle for protection, said Bales' Seattle-based lawyer.

The Bales' off-white, spacious wood home sat dark on Friday night in a neighborhood now filled with news media in the town of Bonney Lake east of Tacoma. A children's playset was in the backyard, situated about a block from Lake Tapps.

Few neighbors were present. At one house backing on to Bales' property, a handwritten sign addressed to media was posted on the door, reading: We don't know Bales, so don't ask.

Beau Britt, staying at his parents' house across the street, said he did not know Bales or his family. It's not the sort of area where you just walk up to a house and start talking to them, Britt said.

SOLDIER WAS ON FOURTH TOUR

Attorney Browne told Reuters that post-traumatic stress disorder, or PTSD, would likely be part of the defense of the four-tour veteran.

It is commonly used in military defense, he said, calling it a mitigating factor. Browne has said the soldier was unhappy about returning to combat after being wounded twice in Iraq.

He added on Friday that the man had witnessed a serious injury to a comrade the day before the massacre in the southern province of Kandahar on Sunday.

One leg was blown off, Browne said, and the sergeant was nearby. Browne or a colleague from the defense team plan to meet the sergeant next week, he said.

The shootings of the 16 villagers have harmed relations between Afghanistan and the United States. Afghan President Hamid Karzai has accused the Pentagon of failing to fully cooperate with an investigation into the killings.

Browne told CNN he had spoken with his client earlier in the morning, and in the short conversation the soldier sounded distant and kind of like a deer in the headlights, but OK.

At a news conference on Thursday, Browne described his client as an exemplary soldier who was upset at having to do a fourth tour of duty in a war zone and was likely suffering from stress after seeing colleagues wounded.

An unnamed U.S. official had told The New York Times the killings were a result of a combination of stress, alcohol and domestic issues — he just snapped.

But Browne said on CNN that marital problems were totally bogus. He said his client had a very strong marriage and, frankly, we're all taking offense at that.

Karzai met with village elders and families of victims of the shootings on Friday and appeared to back their belief that a single gunman could not have killed so many people and in different places some distance apart.

On Thursday, Karzai called for NATO troops to leave Afghan villages and confine themselves to major bases, underscoring fury over the massacre and clouding U.S. plans to withdraw its troops from Afghanistan by the end of 2014.