Reported  by Pierre Thomas, Richard Esposito and Jack Cloherty:

Top U.S. counterterrorism officials told ABC News that they worry about revenge attacks from Anwar al-Awlaki’s followers, and will keep security at “hair-trigger” response for the immediate future.

Before he was killed in a U.S. drone attack, the American-born cleric had enornmous success in recruiting American disciplines online. His anti-American sermons and radical online magazine,  Inspire,  urged violence against Americans -and published the bomb-making recipes that could help make it possible. 

Al-Awlaki was also the commander of arguably the most active terror cell in the world - al Qaeda in the Arabian Peninsula, the group behind the attempted Christmas Day  2009  bombing of a U.S. passenger jet, and the bombing attempt in 2010 of U.S. cargo plane flights out of Yemen. He was linked to 19 U.S. born jihadists through his fiery, inspiring rhetoric.

“We have to be concerned about the potential for retaliatory strikes,” one official said. “Al-Awlaki is a unique figure” and there is now an immediate dual problem to deal with. Al Qaeda in the Arabian Peninsula -the terror organization’s most active division – is going to want to respond, and will at some point.

Federal law enforcement officials are meeting today to discuss the potential for reprisals and a bulletin to U.S. law enforcement is expected to go out as early as tonight. This statement today from New York Police Commissioner Raymond Kelly makes the worry plain.

“We know al-Awlaki had followers in the United States, including New York City, and for that reason we remain alert to the possibility that someone might want to avenge his death.” Kelly said.

Of the 50 Americans charged in homegrown terror cases, 19 have either been directed by, or influenced by, al-Awlaki, Justice Department records show. In addition, a number of recent FBI “sting” operations have snared would-be homegrown terrorists inspired by al-Awlaki. Officials now fear that some of his disciples here in the U.S.  will plot their revenge.

“This is not the end of AQAP, but this is big, this is significant,” said Frank Cilluffo, director  of the Homeland Security Policy Institute at George Washington University. “It is especially significant because of Alwaki’s role in radicalizing and recruiting Westerners.”