The U.S. military said Saturday that an Iraqi soldier apparently shot dead two American service members for "reasons that are as yet unknown" while they were on a joint patrol north of the capital.

Three other U.S. soldiers and one civilian interpreter were wounded in the Dec. 26 attack, the military said in a statement. The shooting occurred as American and Iraqi soldiers were conducting operations to establish a combat outpost in Ninevah province in northern Iraq.The Iraqi soldier who allegedly opened fire fled the scene but was identified by other Iraqi army personnel and was then captured, the military said. Two Iraqi soldiers are being held in connection with the incident.

The U.S. military identified the two Americans killed as Capt. Rowdy Inman and Sgt. Benjamin Portell, both of whom were assigned to the 3rd Squadron, 3rd Armored Cavalry Regiment.Portell, 27, was from Bakersfield, Calif., and Inman, 38, was from Panorama Village, Texas. Both were stationed at Fort Hood.U.S. and Iraqi investigations into the incident are under way, the military said.

Meanwhile, Prime Minister Nouri al-Maliki returned to Iraq after spending a week in London for what his office had described as a routine medical checkup.No official information has been released as to what kind of medical checks he underwent in London. But one of his advisers, speaking on condition of anonymity, said he had traveled to Britain after falling ill, but that the checkup did not reveal any problems.

"I am in good health. I will resume work directly," al-Maliki told reporters at the airport. "We will proceed with our process of rebuilding."At the time of his departure on Dec. 29, an adviser to al-Maliki, Yassin Majeed, had said the prime minister had delayed a previous trip because the "security situation did not allow it."

Security has improved significantly across Iraq in the past six months, although violent attacks still claim dozens of lives each week, and U.S. and Iraqi forces continue to fight insurgents and al-Qaida in Iraq.On Saturday, a roadside bomb explosion struck a passing minibus north of the town of Muqdadiyah, which lies about 60 miles north of Baghdad, killing six people, local authorities said.

The bomb also wounded another three people, said an official in the joint coordination center of Diyala province — which remains one of Iraq's most violent areas. The official spoke on condition of anonymity because he was not authorized to speak to the media.In the province's capital, Baqouba, 35 miles northeast of Baghdad, another roadside bomb wounded three civilians, police said. It was unclear what the target was.

The city has seen several attacks recently, and on Friday a one-day vehicle ban was imposed on the city because of "increased violent events during last week," said Baqouba police chief Brigadier Hasan al-Obaidi. The ban also was aimed at protecting worshippers going to mosques for Friday prayers.

In Baghdad, a roadside bomb struck a passing Iraqi Army patrol in the Azamiyah neighborhood in the northern part of the capital Saturday, wounding four civilians, a police officer said on condition of anonymity because he was not authorized to speak to the media.

Despite continuing daily attacks across the country, violence has fallen significantly in the country — by 60 percent since June, the U.S. military says.One indication of the improvement in security was the reopening of the Samarra dam bridge, one of the entrances into the city 60 miles north of Baghdad, on Thursday, the U.S. military said in a statement.

Entrances into the city had been closed for about eight months due to the violence. "Commerce into and within the city stopped. The reopening of the bridge and other entry points is a direct result of improving security," the statement said.