An Army sergeant opened fire on coalition forces attending a stress clinic at a military base outside of Baghdad International Airport Monday, killing five fellow soldiers, the Pentagon and U.S. Command said.

A senior military official said the soldier had been disarmed after an earlier incident at the center but returned with another weapon. The official spoke on condition of anonymity because the investigation into the shootings was ongoing.

According to the official, the sergeant had been involved in a verbal altercation at the center. His service weapon was taken from him for his own protection and he was driven back to the center later in the day.

When the sergeant returned he had another weapon. It was unclear whether he was returning under orders or of his own volition.

A brief U.S. military statement said the assailant was taken into custody following the 2 p.m. shooting at Camp Liberty, a sprawling U.S. base on the western edge of Baghdad near the city's international airport.

The statement said nobody else was hurt, but a senior defense official in Washington said three people were wounded. The names of the victims and shooter were not released.

RAW DATA: Camp Liberty

Pentagon officials said the shooting happened at a stress clinic, where troops can go for help with the stresses of combat or personal issues. It was unclear whether the gunman and his victims were workers at the clinic or were there for counseling. Soldiers routinely carry weapons on Camp Liberty and other bases, but they are supposed to be unloaded.

President Obama said in a statement that he was "shocked and deeply saddened" by the report, adding that "my heart goes out to the families and friends" of all those involved "in this horrible tragedy."

White House spokesman Robert Gibbs said Monday the shooting at Camp Liberty was a "terrible tragedy" and Obama plans to meet with Defense Secretary Gates later in the day to discuss the matter.

There have been several incidents recently when gunmen dressed as Iraqi soldiers have opened fire on American troops, including an attack in the northern city of Mosul on May 2 when two soldiers and the gunman were killed.

The toll from the Monday shooting was the highest for U.S. personnel in a single attack since April 10, when a suicide truck driver killed five American soldiers with a blast near a police headquarters in Mosul. The U.S. death toll in April was 19, the highest in seven months, amid an upsurge of violence in Iraq.

Separately, the military announced Monday that a U.S. soldier was also killed a day earlier when a roadside bomb exploded near his vehicle in Basra province of southern Baghdad.

Attacks on officers, known as fraggings, were not uncommon during the Vietnam war as morale in the ranks sank.

But the only other member of the U.S. military convicted of murdering a superior since the wars in Iraq and Afghanistan began is Army Sgt. Hasan Akbar of the 101st Airborne Division. Akbar was sentenced to death for a 2003 grenade-and-rifle attack at a base in Kuwait before his unit's move into in Iraq.

Also Monday, a senior Iraqi traffic officer was assassinated Monday morning on his way to work in Baghdad. It was the second attack on a high-ranking traffic police officer in the capital in as many days.

A car cut off Brig. Gen. Abdul-Hussein al-Kadhoumi as he drove through a central square in the capital and a second vehicle pulled up alongside and riddled him with bullets, police said, citing witnesses. Al-Kadhoumi was director of operations for the traffic authority.

The gunmen were armed with pistols equipped with silencers, the police added on condition of anonymity because they were not authorized to speak to the media.

Incidents involving gunmen armed with sophisticated weapons, including silencers, have been on the rise since a string of high-profile robberies in April.

The Associated Press contributed to this report.