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." Watch Rieckhoff and others discuss battle stress »
U.S. Rep. Harry Mitchell, D-Arizona, a member of the House Veterans Affairs Committee, said the military needs to reach out to military personnel who may be suffering from combat stress.
"We simply cannot wait for our men and women serving in the military, or our nation's veterans transitioning back to civilian life, to come to us. We need to go to them," Mitchell said in a statement Monday.
The recession has added another level of stress for soldiers fighting wars in Iraq and Afghanistan, Rieckhoff said.
"You don't just deploy a soldier; you deploy an entire family," he said. "So, you got mothers, brothers, husbands, wives back home who are extremely concerned. They're dealing with a tough economy. All that goes into the stress of the deployed soldier, who's already got enough to worry about in combat."
Monday's shootings occurred at a stress clinic at Camp Liberty, near Baghdad's international airport, two senior defense officials said.
President Obama was shocked by the killings and planned to discuss the issue in a meeting with Defense Secretary Robert Gates, White House spokesman Robert Gibbs said Monday.
It might be difficult for the White House to address the issue because of the number of troops deployed, Rieckhoff said.
"I think they're making the right steps and saying the right things, but they're covering a lot of ground," he said. "They have a lot of catching up to do to get us up to speed and to meet this demand."
Monday's attack marked the sixth incident in which a service member killed a comrade since the launch of Operation Iraqi Freedom, the March 2003 invasion of Iraq, according to military records.
The latest incident was the worst such attack in the six-year-old war, U.S. officials said.