The command also said U.S. soldiers had killed four other suspects a day earlier after coming under fire from machine guns and rocket-propelled grenades in Shiite sections of the capital. The troops seized dozens of rifles and several rounds of ammunition, the statement said.
One of the two al-Qaida suspects, who was captured with four aides in Mosul, is believed to have overseen security for the group's branch in that northern city, the military said. Mosul is one of the terror network's last urban strongholds and the target of a joint U.S.-Iraqi operation.
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The man, who was not identified by name, is also suspected of masterminding bombings against Iraqi police in the area, the U.S. military said.
The other al-Qaida in Iraq suspect was apprehended along with an assistant in Tikrit, a Sunni Arab city south of Mosul. He allegedly helped organize suicide bombings and the movement of foreign fighters into the country, a U.S. statement said.
The suspected Shiite militia leader and five associates surrendered without incident at his home in Kut, southeast of the Iraqi capital, the U.S. said. He was accused of involvement in the murder of Iraqis and American soldiers, it said.
Violence has dropped dramatically in Iraq since a May 11 cease-fire put an end to seven weeks of fighting by U.S. and Iraqi troops against Shiite militias in Baghdad's Sadr City district.
Since then, government forces have intensified security operations to restore control of Sadr City and Basra, the big southern city where Shiite gunmen had ruled the streets for more than three years.