During a routine patrol, U.S. forces also found burned ballot materials inside a Mosul warehouse after a tip by an Iraqi security officer. Efforts are under way to replace the materials for the January elections.
Iraq is slated to hold national elections by Jan. 31 to elect a 275-member assembly in what is expected to be a major step toward building democracy.
Iraqis storm into the Sunni Abu Hanifa mosque in Baghdad following clashes there on Friday.
In Fallujah, battles flared as troops hunted holdout insurgents, and one U.S. Marine and one Iraqi soldier were killed, U.S. officials said.
U.S. troops sweeping through the city west of Baghdad found what appeared to be a key command center of terror mastermind Abu Musab al-Zarqawi, along with a separate workshop where an SUV registered in Texas was being converted into a car bomb and a classroom containing flight plans and instructions on shooting down planes.
Iraqi authorities have acknowledged that al-Zarqawi and other insurgent leaders escaped the invasion of Fallujah.
The U.S. casualty toll in the Fallujah offensive stood at 51 dead and about 425 wounded. An estimated 1,200 insurgents have been killed, with about 1,025 enemy fighters detained, the military said.
In other developments:
- U.S troops arrested a representative of Muqtada al-Sadr near the holy city of Karbala - the second arrest of the radical Shiite cleric's aides in two days, al-Sadr's office said. On Thursday, another al-Sadr aide was arrested in Najaf for speaking out against the U.S.-led assault on Fallujah.
- U.S. troops were conducting an offensive in the northern Iraqi town of Hawija after a recent escalation of violence in the Sunni stronghold wounded three American soldiers and 10 Iraqi National Guards.
- Insurgents struck back in the volatile Sunni area of Haditha, northwest of Fallujah, blowing up the mayor's office and the police command center. Leaflets warned that anyone wearing a police uniform or reporting to a police station "will be killed."
Associated Press Military Writer Robert Burns in Washington contributed to this report.