The arrest of Gadahn is a major victory in the U.S.-led battle against al-Qaida 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-Qaida 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.
In the 25-minute video posted on militant Web sites Sunday, Gadahn described Maj. Nidal Hasan as a pioneer who should be a role model for other Muslims, especially those serving Western militaries.
"Brother Nidal is the ideal role-model for every repentant Muslim in the armies of the unbelievers and apostate regimes," he said.
Gadahn was dressed in white robes and wearing a white turban as he called for attacks on what he described as high-value targets.
"You shouldn't make the mistake of thinking that military bases are the only high-value targets in America and the West. On the contrary, there are countless other strategic places, institutions and installations which, by striking, the Muslim can do major damage," he said, an assault rifle leaning up against a wall next to him.
Hasan has been charged in the Nov. 5 shooting that killed 13 people at Fort Hood, Texas. The 39-year-old Army psychiatrist remains paralyzed from the chest down after being shot by two civilian members of Fort Hood's police force.
In the latest video, Gadahn said those planning attacks did not need to use only firearms. "As the blessed operations of September 11th showed, a little imagination and planning and a limited budget can turn almost anything into a deadly, effective and convenient weapon."
