The man was captured in a raid by U.S.-led coalition forces in Iraq late in 2004, said Matthew Waxman, the deputy assistant secretary of defense for detainee affairs.
"Weapons and bomb-making materials were in his residence at the time he was captured," Waxman said.
Waxman described the man as a personal associate of Zarqawi and an emissary to insurgent groups in several cities in Iraq. Zarqawi, who has declared his allegiance to Osama bin Laden's al Qaeda network, is the most-wanted man in Iraq and is blamed for numerous bombings since the U.S.-led invasion removed Saddam Hussein from power two years ago.
Defense officials also believe the captured American helped coordinate the movement of insurgents and money into Iraq, Waxman said.
The officials said the man holds joint U.S.-Jordanian citizenship but declined to provide his hometown or otherwise identify him.
After his capture, a panel of three U.S. officers determined he was an enemy combatant and not entitled to prisoner-of-war status under the Geneva Convention, Waxman said. He is still being held as a security threat but has been visited by representatives of the International Committee on the Red Cross.
He is the first American known to be captured fighting for the insurgency in Iraq, Waxman said, and officials are considering options how to proceed with his case.
His capture represents a thorny legal issue for the military. It is uncertain whether he will be turned over to the Justice Department for investigation, or to Iraq's new legal system, which has handled the prosecution of other foreign fighters who came to Iraq to fight the U.S.-led occupation and new Iraqi government.
Details on the man's American citizenship were sketchy, but Waxman said he believed the man was born overseas but moved to the United States later. It was unclear when he left.
By John J. Lumpkin