Jeffrey Toobin, CNN senior legal analyst, said on CNN's "The Situation Room" that Holder made it clear in his letter that he would have to examine the facts of each situation separately and advise the president on his legal authority.
"Again, he made a point of saying Senator Rand Paul's question is hypothetical at this time. But at some point down the road, this may not be hypothetical," Toobin said.
Brennan defended the use of drones overseas at his confirmation hearing weeks ago, but acknowledged there should be more public discussion.
In a written response to the intelligence panel, Brennan, too, said the administration has "no intention" of killing Americans with drones in the United States.
The issue of targeting American terror suspects with lethal force was brought into sharp focus in 2011 when New Mexico-born Anwar al-Awlaki was killed in a drone strike in Yemen.
Officials said he played an operational role in al Qaeda in the Arabian Peninsula. Brennan confirmed a connection at his hearing between al-Awlaki and Umar Farouk Abdulmutallab, who tried to blow up a Delta Air Lines jet enroute to Detroit in 2009 with a bomb in his underwear.
The Obama administration has said targeting Americans overseas with drones is only permitted when the U.S. government determines a suspect to be an imminent threat to the United States and when capture would not be feasible. Additionally, all applicable laws must apply.
The Justice Department's Office of Legal Counsel turned over new documents on its legal rationale to the Senate intelligence panel earlier in the day.
Some key members who had been pressing for the information said they were satisfied with the White House cooperation.
"We are pleased that we now have the access that we have long sought and need to conduct the vigilant oversight with which the committee has been charged. We believe that this sets an important precedent for applying our American system of checks and balances to the challenges of 21st century warfare. We look forward to reviewing and discussing these documents in the days ahead," Democrats Ron Wyden of Oregon and Mark Udall of Colorado, and Susan Collins, a Maine Republican, said in a statement.