John Brennan, nominee for CIA Director, arrives at a meeting on Capitol Hill in Washington on January 31, 2013. (REUTERS/Yuri Gripas)

WASHINGTON — Is it right to use unmanned drones to assassinate terrorists even if they are American citizens?

And under what conditions can the U.S. government murder one of its own?

Those are some of the tough questions John Brennan is likely to face from Senators Thursday during his nomination hearing to head up the Central Intelligence Agency.

Brennan, the White House' counter-terrorism chief, is considered the architect of President Barack Obama's drone policy which has killed an estimated 2,100 people, including some U.S. citizens, in Pakistan, Afghanistan, Yemen and North Africa.

Earlier this week, a secret memo was leaked that showed the Obama administration had claimed authority to assassinate U.S. citizens who were part of terrorist organizations who posed an "imminent threat of violent attack" to America.