Conflicting reports emerged Sunday about whether Adam Gadahn, a U.S.-born spokesman for al Qaeda, has been arrested in Pakistan.

A senior Pakistani government official told CNN that Gadahn was arrested Sunday in Karachi, and a second senior Pakistani government official later confirmed the arrest. But a U.S. intelligence official said there appears to be no validity to the reports that Gadahn was in custody, and other U.S. officials also said they have no indication that Gadahn has been captured.

Reports of the arrest came hours after Islamist Web sites posted video of Gadahn praising the November massacre at Fort Hood, Texas. On the video, Gadahn said the U.S. Army major charged with gunning down 13 people "lit a path" for other Muslim service members to follow.

Gadahn, also known as Azzam the American, has routinely posted lengthy videos on Islamist online forums.

In 2006, he was indicted on charges of treason and providing material support to terrorists. The U.S. government has offered a $1 million reward for information leading to his capture.

Authorities have targeted several key Islamist figures in Karachi, the populous port city and financial capital of Pakistan. For some time, it has functioned as a hideout for Taliban and al Qaeda sympathizers.

One of the figures, top Taliban leader Agha Jan Motasim, was arrested on Friday. Motasim's capture came on the heels of the arrest of the Taliban's No. 2 figure and overall military commander, Mullah Abdul Ghani Baradar.

Last month, Taliban and government sources confirmed that Pakistani Taliban leader Hakimullah Mehsud had died. A government official told CNN Mehsud died as a result of a January 14 unmanned drone attack in North Waziristan; other sources said Mehsud died near the city of Multan in central Pakistan while on his way to a treatment center in Karachi.

A city of 13 million -- with some estimates of 100,000 new arrivals a month -- Karachi as seen an influx of Pashtuns from the tribal border region with Afghanistan. Many fled there during fighting and offensives in the Northwest Frontier Province and Waziristan, making it a comfortable place for the Taliban to blend in and count on a network of supporters.

"I think its become apparent ... that al Qaeda doesn't really have a base of sanctuary inside Afghanistan, at least not one where its leaders feel safe," CNN's Senior International Correspondent Nic Robertson said. "They've taken to hiding inside Pakistan."

Gadahn's reported presence in Karachi "is an indication of how easily it is for al Qaeda sympathizers and Taliban sympathizers to hide in that city," Robertson said.

Gadahn, 31, grew up on a California farm and was home-schooled until 17. A year later he moved in with his paternal grandparents, who were secular Jews. He converted to Islam at the Islamic Society of Orange County, California, but was banned from the mosque two years later after hitting its chairman, Haitham Bundjaki.

In 1997 Gadahn began working for a California charity suspected of having ties to al Qaeda. He moved to Pakistan in 1998.

His family has said they last heard from him in 2002. In 2004, the FBI identified him as part of an al Qaeda cell that was planning attacks aimed at disrupting that year's presidential election in the United States.

In October 2004, he began appearing in disguise in al Qaeda videos. Gadahn dropped the disguise in 2006.

In 2008, he renounced his U.S. citizenship and destroyed his passport in another al Qaeda video.

In his video message posted online Sunday, Gadahn says Muslims should emulate the alleged Fort Hood shooter.

"I believe that defiant Brother Nidal is the ideal role model for every repentant Muslim in the armies of the unbelievers and apostate regimes," Adam Gadahn says in English in the video.

Maj. Nidal Hasan, an Army psychiatrist and a U.S.-born citizen, is charged with 13 counts of premeditated murder in the November 5 killings. Hasan is also facing 32 counts of attempted premeditated murder and is eligible for the death penalty.

"The Mujahid brother Nidal Hasan is a pioneer, a trailblazer and a role model who has opened a door, lit a path and shown the way forward for every Muslim who finds himself among the unbelievers and yearns to discharge his duty to Allah and play a part in the defense of Islam and Muslims," Gadahn says in the video.

Gadahn also cites in Sunday's video the U.S. and allied buildup in Afghanistan, where the United States is in the process of adding about 30,000 troops.

"It is rapidly becoming clear that this already-hot global battle is about to get even hotter," he says. "This is a war which knows no international borders and no single battleground, and that's why I am calling on every honest and vigilant Muslim in the countries of the Zionist-Crusader alliance in general and America, Britain and Israel in particular to prepare to play his due role in responding to and repelling the aggression of the enemies of Islam."

In December, Gadahn released a video message in English offering condolences to "unintended Muslim victims" killed in attacks in Afghanistan, Pakistan and elsewhere. It was a rare example of al Qaeda offering condolences to the families of those killed in the group's own attacks.