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U.S. military sources tell Fox News the American soldier accused of killing 16 Afghan civilians last weekend is Staff Sgt. Robert Bales.

He arrived at the maximum-security military prison at Fort Leavenworth in Kansas Friday, Fox News confirms.

Military officials have declined to identify the suspect publicly, insisting that it is usual procedure to keep a suspect's identity secret until he is officially charged. They have maintained that stance even after a hearing for the detained soldier Tuesday found probable cause to continue holding him, and he was sent from Afghanistan to a detention facility in Kuwait.

The soldier was being flown Friday to the U.S. military's only maximum-security prison, at Fort Leavenworth, Kansas, sources said. The move to the U.S. does not necessarily mean an announcement of formal criminal charges is imminent, a defense official told The Associated Press.

Bales is a 38-year-old staff sergeant, husband, father of two young children and a veteran who was in the midst of his fourth tour in a war zone.

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John Henry Browne, a defense attorney from Seattle, confirmed his client's identity.

Bales was featured in a brief article in a September 2011 military newsletter detailing a training exercise at Fort Irwin meant to simulate efforts to establish relationships with Afghan village residents.

"How's the security affecting your family?" the article quotes Bales asking a village elder. The elder replies it's "much better than yesterday."

Another article, from 2009, quotes Bales describing a particularly intense firefight in Iraq: "It was like a match lit up," Bales said. "It looked like a toy with a candle lit."

Browne said the sergeant is originally from the Midwest but now lives near Joint Base Lewis-McChord in Washington state. His children are 3 and 4.

The sergeant's family says they saw no signs of aggression or anger. "They were totally shocked" by accounts of the massacre, Browne said. "He's never said anything antagonistic about Muslims. He's in general very mild-mannered."

Browne, who said he has met with the family and talked with the suspect, cited a need to protect family members in declining to release the soldier's name.

Reporters swarmed Bales' neighborhood in Lake Tapps, in Washington state on Friday night in the rural community, a wooded area filled with pine trees about 20 miles northeast of the base.

Kassie Holland, who lives next door, said she would often see Bales playing with his two kids and the family together at the modern split-level home.

"My reaction is that I'm shocked," she said. "I can't believe it was him. There were no signs. It's really sad. I don't want to believe that he did it."

"He always had a good attitude about being in the service. He was never really angry about it. When I heard him talk, he said, it seemed like, yeah, that's my job. That's what I do. He never expressed a lot of emotion toward it."

The soldier, said to have received sniper training, is assigned to the 2nd Battalion, 3rd Infantry Regiment of the 3rd Stryker Brigade Combat Team, of the 2nd Infantry Division, which is based at Lewis-McChord and has been dispatched to Iraq three times since 2003, military officials say.

During the suspect's time in Iraq, Browne said, the soldier was injured twice. A battle-related injury required surgery to remove part of one foot, the lawyer said.

But Browne and government officials differ in their portrayal of a second injury, to the soldier's head, in a vehicle accident.

A government official said this week that the accident was not related to combat. But Browne said the man suffered a concussion in an accident caused by an improvised explosive device.

Browne also said his client was "highly decorated," but did not provide any specifics.

Bales completed 20 hours of anger-management counseling following a 2002 arrest at a Tacoma hotel for investigation of assault. Browne said the case involved a woman who was not his wife. It was not immediately clear how long Bales has been married to his wife.

Tacoma Municipal court administrator Yvonne Pettus provided a copy of the court docket, but said clerks could not immediately locate the case file, which is either in archives or destroyed. The docket shows that Bales pleaded not guilty, underwent the 20 hours of anger management treatment, and the case was dismissed.

When he returned to the Seattle area, the staff sergeant at first thought he would not be required to join his unit when it shipped out for Afghanistan, the lawyer said. His family thought he was done fighting and was counting on him staying home. Until orders came dispatching him to Afghanistan, he was training to be a military recruiter, Browne said.

"He wasn't thrilled about going on another deployment," Browne said. "He was told he wasn't going back, and then he was told he was going."

The staff sergeant arrived in Afghanistan in December. On Feb. 1 he was assigned to a base in the Panjwai District, near Kandahar, to work with a village stability force that pairs special operations troops with villagers to help provide neighborhood security.

On Saturday, the day before the shooting spree, Browne said, the soldier saw his friend's leg blown off. Browne said his client's family provided him with that information, which has not been verified.

The other soldier's "leg was blown off, and my client was standing next to him," he said.

Browne said he did not know if his client had been suffering from PTSD, but said it could be an issue at trial if experts believe it's relevant. Experts on PTSD said witnessing the injury of a fellow soldier and the suspect's own previous injuries put him at risk.

The lawyer denied reports that the soldier had marital problems, saying he and his wife have a solid relationship.

The identification of the shooter quickly elicited a variety of reactions throughout Twitter. Some used social media to condemn Bales and called him a terrorist. Others called him a war hero who should not have been deployed to Afghanistan after serving three tours in Iraq. Others offered prayers for him and his family.

Fox News' Jennifer Griffin and Justin Fishel and The Associated Press contributed to this report.