BAGHDAD, Iraq (CNN) -- A U.S. soldier fired on his fellow troops at a counseling center at a base outside Baghdad on Monday, U.S. officials said, killing five people in the worst such attack of the six-year war.
U.S. Army troops get a safety briefing before departing Camp Liberty, Iraq, in December 2008.

U.S. Army troops get a safety briefing before departing Camp Liberty, Iraq, in December 2008.

The shooting occurred at 2 p.m. at a stress clinic at Camp Liberty, near Baghdad's international airport, two senior defense officials said. Though initial reports indicated the attacker was killed in the incident, the U.S. command in Baghdad said late Monday that a suspect in the killings was in custody.

"Any time we lose one of our own, it affects us all," Col. John Robinson, a U.S. military spokesman, said in a written statement. "Our hearts go out to the families and friends of all the service members involved in this terrible tragedy."

Neither the suspect nor any of the victims had been identified, but a defense official with access to the latest reports on the incident told CNN that the suspect had been a patient at the treatment center.

"Preliminary reports show the soldier was was being escorted to the clinic, for reasons not yet explained," an Army official, who spoke on condition of anonymity, told the Army Times.

"Once inside, he got into a verbal altercation with the staff and was asked to leave. The soldier and his escort got back into their vehicle and began to drive away.

"At some point during the drive, the soldier got control of his escort's weapon and ordered the escort out of the vehicle," the official said, according to the Times. "The soldier then drove back to the clinic, walked in and began shooting."

In Washington, the White House said President Obama was shocked by the attack.

"The president's heart goes out to the families and friends of all the service members involved in this horrible tragedy," White House spokesman Robert Gibbs told reporters.

"He was shocked by the news of this incident and will press to ensure that we fully understand what happened at the clinic, and that we are doing everything we can to ensure that our men and women in uniform are protected."

The president planned to bring up the issue in a meeting with Defense Secretary Robert Gates, Gibbs said. Gates expressed his own "horror and deep regret" Monday afternoon. Video Watch the defense secretary talk about the incident »

"We are still in the process of gathering information on exactly what happened," Gates said. "But if the preliminary reports are confirmed, such a tragic loss of life at the hands of our own forces is a cause for great and urgent concern. And I can assure you that it will get this department's highest-priority attention."

Camp Liberty is tightly guarded, and U.S. troops are required to clear their weapons of ammunition while on the base. The only service members who have loaded weapons are those guarding high-ranking officers and military police.

Monday's attack marks the sixth incident in which a service member was killed by a fellow service member since the launch of Operation Iraqi Freedom, the March 2003 invasion of Iraq.

"The events at Camp Liberty today are a great tragedy. The friends and families of those killed have suffered a terrible loss, and our thoughts are with them during this difficult time.

Unlike during the Vietnam War, today’s military is a professional, all-volunteer force. There have been only five cases of intentional fratricide by U.S. service members in Iraq. But these incidents, however rare, draw public attention to an important issue: the enormous stress on our armed forces. Many troops are under great psychological strain and are not receiving the treatment they need. Over 600,000 troops have served more than one combat tour since 9/11. Military suicide rates have hit record highs every year since 2003. Much more must be done to address troops’ psychological injuries before they reach a crisis point.”

Iraq and Afghanistan Veterans of America is honored to offer our full support behind S. 711, the Post-Deployment Health Assessment Act (PDHAA) of 2009, which will require in-person mental health screenings for deploying servicemembers. Mandatory face to face screenings for all returning combat veterans is one of IAVA’s top legislative priorities and we believe that the PDHAA will cast a wide and sturdy safety net for our returning heroes.

Research has only verified what every returning veteran already knows: that no one goes to combat and comes home unchanged. 20% of all veterans who have deployed to Iraq and Afghanistan have screened positive for combat stress injuries, and less than half of those who suffer from PTSD have been treated.

As a country we must act swiftly and decisively to ensure that every returning veteran gets an opportunity to fully reintegrate. This legislation would require multiple in-person mental health screenings for all returning servicemembers, before a deployment, post-deployment and every year following the deployment. This screening would identify symptoms of combat stress injuries, such as Post Traumatic Stress Disorder (PTSD), depression and anxiety. With the surge of veterans returning from Iraq and Afghanistan, we have an opportunity with this legislation to identify those in need and help them access the care they require.