BAGHDAD, Iraq (CNN) -- A U.S. soldier fired on his fellow troops at a counseling center at a base outside Baghdad, Iraq, on Monday, U.S. officials said, killing five people in the worst such attack of the 6-year-old war.
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 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 a suspect in the killings was in custody.
Neither the suspect nor any of the victims had been identified, and it was unclear whether the attacker had any connection to the clinic.
"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."
"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. And Gates expressed his own "horror and deep regret" Monday afternoon. 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."
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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.
In March 2003, Capt. Christopher Seifert and Maj. Gregory Stone of the Army's 101st Airborne Division were killed in a grenade attack at Camp Pennsylvania, Kuwait, that wounded 14 other officers. Sgt. Hasan Akbar was convicted by a court-martial in 2005 and sentenced to death.
Sgt. Joseph Tackett was fatally shot in June 2005 by a fellow soldier in Baghdad.
The same month, Capt. Phillip Esposito and Lt. Louis Allen were killed in an explosion at a base in Tikrit, north of Baghdad. The military charged a sergeant in their company, Alberto Martinez, with murder in their deaths, but a military jury acquitted him in 2008.
Camp Liberty is part of the Camp Victory Complex, one of the largest U.S. military bases in Iraq. It lies just northeast of Baghdad International Airport and is also near the massive al-Faw palace of executed former dictator Saddam Hussein.
Once known as Camp Victory North, the base was renamed Camp Liberty in September 2004, or Camp Al-Tahreer in Arabic, according to GlobalSecurity.org, a Web site that provides background on military and security issues.
Like other American facilities in Iraq, Camp Liberty provides a host of amenities for the thousands of U.S. troops who call it home during their combat tours.
The base resembles a giant trailer park, dotted with air-conditioned two- or three-person units that house thousands of service members. It boasts several dining facilities catered by a private company, fully equipped gymnasiums, recreation centers and a post exchange that rivals Wal-Mart.
The military set up several chapels, and each unit operates stress-relief clinics -- such as the one at which the shootings occurred Monday -- where troops can request counseling.