Can the CIA legally kill a prisoner?

The New Yorker's investigation into the CIA's crucifixion-style killing of an Iraqi detainee called Manadel al-Jamadi is subtitled Can the C.I.A. legally kill a prisoner? :

In August, 2002, the US Justice Department’s Office of Legal Counsel sent a memo on interrogations to the White House, which argued that a coercive technique was torture only when it induced pain equivalent to what a person experiencing death
or organ failure might suffer.

By implication, all lesser forms of physical and psychological mistreatment—what critics have called “torture lite”—were legal.

Pulling people's fingernails out, and 'waterboarding' (almost drowning), are therefore legal according to to the US Justice Department.

It has also ruled that Congress has no constitutional right to interfere with the President's wartime powers to fight the enemy however he sees fit.

It has also dismissed most US national, and International (meaning the Geneva Conventions), regulations on the treatment of prisoners.

The Economist has an excellent editorial on the incalculable damage Bush and Cheney's defence of these practices is doing to the USA's image around the world.

Meanwhile, today's Sunday Times 'Focus' feature suggests that while Britian has taken a different route and does not torture, it still 'does make use of information gleaned through torture perpetrated by others'.