Yes, the president does have the authority to use military force against American citizens on US soil—but only in "an extraordinary circumstance," Attorney General Eric Holder said in a letter to Senator Rand Paul (R-Ky.) Tuesday.
Last month, Paul threatened to filibuster the nomination of John Brennan, Obama's pick to head the CIA, "until he answers the question of whether or not the President can kill American citizens through the drone strike program on U.S. soil." Tuesday, Brennan told Paul that "the agency I have been nominated to lead does not conduct lethal operations inside the United States—nor does it have any authority to do so." Brennan said that the Justice Department would answer Paul's question about whether Americans could be targeted for lethal strikes on US soil.