BAGHDAD , Iraq -LRB- CNN -RRB- -- 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 six-year-old Iraq 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 , 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 . `` 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 . President Obama said that he was `` shocked and deeply saddened '' by the `` horrible tragedy . '' `` My heart goes out to the families and friends of all the service members involved , '' he said in a written statement . `` I will press to ensure that we fully understand what led to this tragedy , and that we are doing everything we can to ensure that our men and women in uniform are protected . '' The president said he discussed the issue in a meeting with Defense Secretary Robert Gates on Monday afternoon . And Gates expressed his own `` horror and deep regret '' at a press briefing . Watch the defense secretary talk about the incident '' `` 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 , '' Gates said . `` And I can assure you that it will get this department 's highest priority attention . '' Adm. Michael Mullen , chairman of the Joint Chiefs of Staff , added the incident shows a need to `` redouble '' efforts regarding troop stress levels . 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 . 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 . In recent years , a number of fast food restaurants such as Burger King and Pizza Hut have opened , as well as private stores and a bazaar that sells local wares . 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 . CNN 's Cal Perry , Chris Lawrence and Mike Mount contributed to this report .