The investigation into the killings of 17 Afghan civilians earlier this month now suggests that the killings happened in two separate waves.

A senior U.S. official confirmed to Fox News that Staff Sgt. Robert Bales, the suspect in the killings, might have carried out one attack, returned to base, and then carried out another.

The lawyer for Army Staff Sgt. Robert Bales said Tuesday the government may have a difficult time proving its case against his client in the massacre of Afghan civilians because “there’s not a crime scene, there's not evidence."

Bales' attorney, John Henry Browne said earlier this week that he doesn't "know really what the facts are and neither do you,” in regard to the details of the slayings. 

“This is a very unusual case because there's not a crime scene, there's not evidence. It's just a really unusual situation," Browne said.

He suggested the stress of war and extended battle could be partly to blame for what happened, but said he needs to learn more before he can begin to fashion a defense.

He hit that same theme earlier on Tuesday when talking to CNN about the shooting deaths of nine children, three women and four men in a village in southern Afghanistan: "I don't know if the government is going to prove much. There's no forensic evidence, there's no confessions."

He added, "Nothing really justifies killing women and children in a noncombat situation. But there may be explanations if that's true. And right now I want to say once again, I'm not sure that's true."

Bales, who had served three tours of duty in Iraq before being sent to Afghanistan, may have been sufferingpost-traumatic stress disorder, Brown said. Bales suffered a traumatic brain injury during a roadside bomb explosion and lost part of his foot in separate tours in Iraq, his attorney has said.

 "Anybody that has seen what he's seen and done what he's done at the request of the military -- and I'm not talking about these allegations -- I think would have PTSD ... Dragging parts of bodies around is not something you forget very often."

He said he expects his client to be charged on Thursday, probably with "homicide and a bunch of other charges," and predicted that the case would last two years.

Browne also denied that the soldier was drunk at the time of the shootings. "A couple of sips of somebody else's bottle," he said. "But that's not drinking."

He said he had met with Bales had met with three of his lawyers, including Browne, for 11 hours and that Bales was in shock and doesn't remember what happened. "That's common with concussive injuries," he said. 'You remember certain things; you don't remember other things."

The attorney said he will not pursue an insanity defense for his client, but one of diminished capacity, CBS reported.