“There are, in my mind, dozens of U.S. persons who are in different parts of the world, and they are very concerning to us,” said John O. Brennan, deputy White House national security adviser for homeland security and counterterrorism.

In a wide-ranging interview with The Washington Times, Mr. Brennan said he would not talk about lists of targeted American terrorists. However, U.S. intelligence and law enforcement agencies have been tracking down U.S. nationals and U.S. passport holders who pose security threats, like the Yemen-based al Qaeda cleric Anwar al-Awlaki, he said.

“They are concerning to us, not just because of the passport they hold, but because they understand our operational environment here, they bring with them certain skills, whether it be language skills or familiarity with potential targets, and they are very worrisome, and we are determined to take away their ability to assist with terrorist attacks,” Mr. Brennan said.