Can the President Order a Killing on U.S. Soil?

In the latest twist in the debate over presidential powers, a Justice Department official suggested that in certain circumstances, the president might have the power to order the killing of terrorist suspects inside the United States

From Newsweek Politics – MSNBC.com

I’m a bit wary about the slippery slope of this argument. If you can order torture, why not order a killing? Where are the limits drawn? Can anything be justified?