The US Army's inspector general is investigating whether psychiatrists threw out diagnoses of post-traumatic stress for soldiers to save money, the civilian head of the army told lawmakers Wednesday.

The review coincides with renewed concern over the psychological strain on American soldiers as military prosecutors investigate Staff Sergeant Robert Bales for a shooting rampage in Afghanistan that left 16 civilians dead.

The move was ordered after the Army's surgeon general, Lieutenant General Patricia Horoho, learned that a forensic psychiatry unit at Joint Base Lewis-McChord in Washington state reversed or revised post-traumatic stress disorder diagnoses for about 40 percent of troops who had been previously assessed with PTSD, officials said.

The suspected gunman in the Afghan shooting spree, Bales, was from a unit out of the Lewis-McChord base but it was unclear if he was among those evaluated by the psychiatrists.

"The surgeon general has asked the inspector general of the army to go and examine all of the similar facilities and locations," Army Secretary John McHugh told the Senate Appropriations subcommittee on defense.

"To this point, we don't see any evidence of this being systemic," he said.

McHugh added that the review would examine whether a standard protocol for mental health diagnoses was being followed and "equally important, that fiscal considerations are not in any way a part of the evaluation. It's simply unacceptable."

Senator Patty Murray, who raised the issue with McHugh, said the numbers at the Lewis-McChord base were alarming.

"Now, in light of all the tragedies that we have seen stem from the untreated invisible wounds of war today, I'm sure that you would agree that this is very concerning," said Murray, who represents Washington state in the Senate.

"And not only is it damaging for our soldiers, but it also really furthers the stigma for others... whether they're deciding to seek help or not today," she said.

After ten years of war, the US military is struggling with high rates of suicide, traumatic brain injuries, post-traumatic stress and other mental health problems among troops who have faced repeated combat tours.