BAGHDAD, May 11 -- An American soldier opened fire on comrades Monday on a large military base in Baghdad, killing five and wounding three, the U.S. military said.

The shooting at Camp Liberty, one of the largest bases in Baghdad, occurred about 2 p.m.

Lt. Col. Brian Tribus, a U.S. military spokesman, said the gunman was taken into custody.

Defense Secretary Robert M. Gates, in a briefing with reporters in Washington, expressed "deep regret" for the shooting. He said, "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."

White House press secretary Robert Gibbs said that President Obama was "saddened" and "shocked" by the shootings and that the president's "heart goes out" to the families of those killed.

Obama will press Gates in a meeting Monday afternoon to investigate the shootings to "ensure we fully understand what happened," Gibbs said.

A U.S. military officer in Baghdad said the shooting occurred at the base's combat stress clinic.

The official, speaking on condition of anonymity, said the incident shook up soldiers, many of whom are in their third and even fourth tours. Some broke down in tears, he said.

"A lot of soldiers are wondering why," the official said. "We will be asking as leaders: What could we have done? How could have we protected the soldiers?"

Most military facilities in Iraq have combat stress clinics, where soldiers seek counseling and are at times prescribed medicine for anxiety and depression.

The Army is grappling with a growing incidence of suicide cases, which military leaders attribute to the stress inflicted by multiple deployments to Iraq and Afghanistan.

A Defense Department official in Washington said three people were wounded, but he did not know whether they were military or civilians.

"It's some form of isolated incident, an unfortunate one at that," he said.

The military did not immediately say what the motive might have been.

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

The incident was among the deadliest attacks for U.S. troops in recent months. It appears to be the deadliest incident in which U.S. deaths were caused by a fellow U.S. soldier since the 2003 U.S. invasion of Iraq.

The shooting was particularly chilling for soldiers based at Victory Base Compound, which includes Liberty, because it is regarded as one of the safest installations for U.S. troops in Iraq.

Control to the compound is tightly restricted, but American soldiers carry weapons on base.

Also on Monday, the military said an American soldier was killed by a roadside bomb in southern Iraq. The attack occurred Sunday at 2 p.m. in Basra province. U.S. soldiers recently deployed additional troops to the province to replace British troops, who formally ended their mission there last month.

Liberty is one of three U.S. military bases adjacent to Baghdad International Airport.

Staff writers Michael D. Shear and Ann Scott Tyson in Washington contributed to this report.

www.washingtonpost.com/wp-dyn/content/article/2009/05/11/AR2009051100265.html