Beginning in 2008 Awlaki began an e-mail correspondence with Nidal Hassan in which the Army doctor asked for and received religious justifications for murder. The two men exchanged some 20 e-mails, according to government investigators.
In one such e-mail, Hassan wrote Awlaki: "I can't wait to join you" in the afterlife.
Following the Fort Hood attacks Awlaki called Hassan a "hero" on his Web site.
"The only way a Muslim could Islamically justify serving as a soldier in the U.S. Army," he wrote, "is if his intention is to follow the footsteps of men like Nidal."
On Monday, the government of Yemen confirmed that failed bomber Abdulmutallab had visited that county in early 2009, raising suspicions that the would-be terrorist met with his spiritual leader.
"It appears that just like with Major Hasan, Awlaki played a role in this," Rep. Pete Hoekstra, R-Mich., ranking minority member of the House Intelligence Committee, told ABC News' "The Blotter."
The congressman told "The Blotter" that he would investigate how Abdulmutallab was radicalized, and whether he went to Yemen on his own initiative to meet members of al Qaeda.
Many of the young men who find Awlaki's message of murder and martyrdom inspiring are disaffected youths, who often feel alienated not just in the countries in which they live but within their own families, said Anthony Cordesman, an expert at the Center for Strategic and International Studies.
"These are young men looking for something and who can be easily persuaded. It is difficult to know how many are found through the appeal of the message and a charismatic leader, versus structured recruiting," Cordesman said.
"As a native English speaker born in the U.S., it makes it easier to reach out to Americans, and the host of other Muslims alienated by living in the West," he said.
Awlaki was born in New Mexico in 1971 while his father went to college. His family moved back to Yemen for a time, but Awlaki returned in 1991 to study engineering at Colorado State University.
Following his graduation, he became an imam, leading mosques in Fort Collins, Colo., San Diego and later outside Washington, D.C.
According to a Roanoke, Va., newspaper, Awlaki presided over the funeral of Nidal Hassan's mother while in Virginia.
After 9/11, authorities looked for a reason to legally detain and question the fiery preacher who was known to have met with two of the hijackers.
Awlaki was arrested twice in the 1990s for soliciting prostitutes, and federal authorities in 2002 hoped to catch and arrest him for violations of the Mann Act, a law that prohibits the transportation of people across state lines for illegal purposes.
Though born in New Mexico, Awlaki claimed in 2002 that he was born in Yemen when applying for a visa to attend college in Colorado -- an alleged incidence of passport fraud.
A warrant was issued for his arrest, but one day before Awlaki was detained in an airport on his way to Yemen, investigators learned that an assistant district attorney had rescinded the warrant.
U.S. Attorney David Gaouette defended the decision to rescind a 2002 felony arrest warrant for Awlaki, telling ABC News' "The Blotter" that his office determined there was insufficient evidence to pursue the case.