John Henry Browne said there were “no forensic evidence” against his client and “no confession”.
He also dismissed reports suggesting Robert Bales was having financial troubles as irrelevant to the case.
Robert Bales, 38 is being held a military detention centre awaiting charges, which are expected this week.
The killings have undermined US relations with Kabul and led to calls for NATO to speed up their planned withdrawal of troops from Afghanistan.
After meeting with Sgt. Robert Bales at a US army base in Kansas, John Henry Browne told reporters: “We’ve all heard the allegations. I don’t know that the government has proved much.”
Robert Bales is the only known suspect in the killings – despite repeated Afghan assertions that more than one American was involved.
The lawyer also responded to questions about Robert Bales’ financial history.
Robert Bales and his wife had reportedly struggled to make the payments on two properties they had bought.
It has now also emerged that – along with another man and his company – Robert Bales owed a reported $1.5 million from an arbitration ruling nearly a decade ago which found him guilty of securities fraud while he was working as a stockbroker.
John Henry Browne told Associated Press “that doesn’t mean anything”.
“Sure, there are financial problems. I have financial problems. Ninety-nine percent of America has financial problems,” John Henry Browne said.
“You don’t go kill women and children because you have financial problems.”
Robert Bales’ wife, Karilyn, has issued a statement expressing her condolences to the victims and their families and saying what reportedly took place is “completely out of character of the man I know and admire”.
John Henry Browne first met his client at Fort Leavenworth on Monday to begin preparing his defense.
The Pentagon has previously said that Sgt. Robert Bales could face charges that carry a possible death penalty.
Such a trial could take years, contrasting with Afghan demands for swift and decisive justice.
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