U.S. soldiers join hands in prayer before a patrol in Iraq last year.

CNN

The killing of five comrades by a U.S. soldier on Monday in Iraq is no surprise and illustrates the mental toll that the current wars take on troops, the leader of a veterans group said.

"It's tragic. I mean, It's deeply disturbing, but I don't think folks who have been in the [war] theater are surprised," said Paul Rieckhoff, executive director of Iraq and Afghanistan Veterans of America.

Rieckhoff talked with CNN's Anderson Cooper on Monday night about the killings.

The "unprecedented" number of times that soldiers are redeployed to Iraq and Afghanistan adds to the stress soldiers are feeling, Rieckhoff said.

"There's a study of one in four folks coming back [from war] with some kind of stress-related mental health injury. But these folks are going back over and over again," he said. "Each time you're deployed, you're more likely to have a mental health disability. There's not enough psychologists, psychiatrists in theater."

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