On Thursday, Senate Judiciary Committee Chairman Patrick Leahy, D-Vermont, and Ranking Member Chuck Grassley, R-Iowa, sent a letter to Obama calling on him to share with the committee any legal memos on the targeted killings of Americans abroad.

"The deliberate killing of a United States citizen pursuant to a targeted operation authorized or aided by our Government raises significant constitutional and legal concerns that fall squarely within the jurisdiction of the Committee," they wrote.

One of the questions the committee submitted to Brennan in advance of the hearing asked how it was determined that an individual was associated with al Qaeda and that a threat was imminent.

Brennan responded in writing that such decisions were made on a "case-by-case basis" in a process involving other agencies.

The White House said this week that questions on the matter have been weighed against legal concerns and discussed publicly.

The Supreme Court has held that the military may use force against an American who is part of enemy forces.

Still civil liberties and other groups want more answers.

Amnesty International said Congress should grill Brennan on his claim that the Obama administration's drone strikes are "conducted in full compliance with the law."

Other controversies at hand

Brennan's chances to lead the CIA at the start of Obama's first term were scuttled by questions about the enhanced interrogations of terrorist suspects.

At Thursday's hearing, Brennan said he had raised objections with colleagues to the enhanced interrogation techniques, but denied any role in managing or enabling their use.

Asked if the techniques had yielded intelligence that saved lives, Brennan did not answer directly. Instead, he repeated his insistence that such techniques would never be used under his watch.

Senate lawmakers also asked about Brennan's role in administration leaks about covert operations, including a foiled al Qaeda bomb plot in Yemen involving a mole.

He denied any wrongdoing, saying that he had gotten involved only after the leaks became public and that he tried to stem a "hemorrhaging" of leaked information in the matter.

Brennan acknowledged in his written responses to committee questions that he voluntarily was interviewed by prosecutors about two investigations. He said in both cases his counsel told him he was being interviewed as a witness, not a target.

A powerful figure at White House

As CIA director, Brennan would report to James Clapper, the director of national intelligence. But he would also have a direct path to the president, talking to him on the phone or briefing him personally.