The arrest of Gadahn is a major victory in the U.S.-led battle against al-Qaeda and will be taken as a sign that Pakistan is cooperating more fully with Washington. It follows the recent detentions of several Afghan Taliban commanders in Karachi.
Gadahn was arrested in the sprawling southern metropolis in recent days, two officers who took part in the operation said. A senior government official also confirmed the arrest.
They spoke on condition of anonymity because they were not authorized to release the information.
Gadahn, who is also known by various aliases, including Yahya Majadin Adams and Azzam al-Amriki, grew up on a goat farm in Riverside County, California, and converted to Islam at a mosque in nearby Orange County.
Gadahn moved to Pakistan in 1998, according to the FBI, and is said to have attended an al-Qaeda training camp six years later, serving as a translator and consultant for the group. He has been wanted by the FBI since 2004, and there is a $1 million reward for information leading to his arrest or conviction.
He has posted videos and messages calling for the destruction of the West and for strikes against targets in the United States. The most recent was posted Sunday, praising the U.S. Army major charged with killing 13 people in Fort Hood, Texas.
A U.S. court charged Gadahn with treason in 2006, making him the first American to face such a charge in more than 50 years. He could face the death penalty if convicted. He was also charged with two counts of providing material support to a designated foreign terrorist organization. (continues here)