WASHINGTON — In the weeks after the Sept. 11 attacks, the eloquent 30-year-old imam of a mosque outside Washington became a go-to Muslim cleric for reporters scrambling to explain Islam. He condemned the mass murder, invited television crews to follow him around and patiently explained the rituals of his religion.

“We came here to build, not to destroy,” the cleric, Anwar al-Awlaki, said in a sermon. “We are the bridge between Americans and one billion Muslims worldwide.”

At first glance, it seemed plausible that this lanky, ambitious man, with the scholarly wire-rims and equal command of English and Arabic, could indeed be such a bridge. CD sets of his engaging lectures on the Prophet Muhammad were in thousands of Muslim homes. American-born, he had a sense of humor, loved deep-sea fishing, had dabbled in get-rich-quick investment schemes and dropped references to “Joe Sixpack” into his sermons. A few weeks before the attacks he had preached in the United States Capitol.

Nine years later, from his hide-out in Yemen, Mr. Awlaki has declared war on the United States.

“America as a whole has turned into a nation of evil,” he said in a statement posted on extremist Web sites in March. Though he had spent 21 of his 39 years in the United States, he added, “I eventually came to the conclusion that jihad against America is binding upon myself, just as it is binding on every other able Muslim.”