A history of mental instability and crazed Internet ramblings suggest Jared Lee Loughner, the alleged Arizona shooter who killed six people on Saturday, wasn't all there. But he probably won't get off by claiming he was insane, say prosecutors and forensic scientists. Evidence of Loughner's premeditation—a string of written and face-to-face interactions with Gifford, a note referring to his "planning ahead," and calling his plot an "assassination"—will make it virtually impossible to argue he was unaware his actions were criminal. "It's very hard to prove insanity at trial," one criminal defense attorney said. "You really have to prove that your mental illness is so severe that you don't even understand that you're committing a criminal act. And it's almost impossible to prove that." Loughner's lawyers have not said how they plan to approach the defense.