John Henry Browne said there was "no forensic evidence" against Staff Sgt Robert Bales and "no confession".
He also dismissed reports suggesting Sgt Bales, 38, was having financial troubles as irrelevant to the case.
Sgt Bales 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 Bales at a US army base in Kansas, Mr Browne told reporters: "We've all heard the allegations. I don't know that the government has proved much."
Staff Sgt Robert Bales
- Aged 38
- Two children
- Enlisted in 2001
- Three previous tours to Iraq
- Injured twice
Mr Browne said he now plans to travel to Afghanistan to gather his own evidence.
The lawyer also responded to questions about Sgt Bales' financial history.
He 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 - Sgt Bales owed a reported $1.5m (£950,000) from an arbitration ruling nearly a decade ago which found him guilty of securities fraud while he was working as a stockbroker.
Mr 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," he said.
"You don't go kill women and children because you have financial problems."
His 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".
Mr Browne first met his client at Fort Leavenworth on Monday to begin preparing his defence.
The Pentagon has previously said that Sgt 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.