TUCSON - In the days and weeks before the shooting rampage in Tucson, Jared Lee Loughner surfed the Internet on his home computer in what investigators believe was preparation for his alleged assassination attempt, law enforcement sources familiar with the investigation said.

The suspect's focus on several Web sites - covering lethal injection, solitary confinement and political assassinations - could have dramatic implications as prosecutors build their case against him on federal murder charges that could carry the death penalty. They hope to use the computer information, along with notes seized in Loughner's home, to show that his acts were premeditated and that he knew right from wrong, the sources said.

The computer analysis, which was completed last week, was turned over to Loughner's attorney Judy Clarke at a court hearing last week, along with notes from 250 interviews. Clarke, who has also represented Unabomber Ted Kaczynski and other high-profile defendants, did not indicate whether she intends to pursue an insanity defense.

Police seized Loughner's computer when they searched his family home in Tucson on Jan. 8, shortly after the shooting outside a Safeway that killed six people and injured 13, including Rep. Gabrielle Giffords (D-Ariz.). Loughner, 22, has entered a plea of not guilty in connection with federal charges.

"The impression investigators have is that he was trying to educate himself on assassinations and also research the consequences," said one source close to the investigation, who spoke on the condition of anonymity because the investigation is ongoing.

Legal experts say evidence from the computer hard drives would pose a significant hurdle in pursuing an insanity defense.

"It indicates he not only knew right from wrong, but he also knew shooting someone, assassinating someone, carried significant consequences," said John Zwerling, an Arlington County defense lawyer who is not involved in the Loughner case. "And it also adds to the premeditation aspect to this. What it shows is he wanted to do a political act and was willing to pay the consequences."

Zwerling said that a jury could be swayed if the defense shows that Loughner had a brain injury or abnormality that rendered him mentally incompetent.

Washington lawyer Joseph diGenova, who as a U.S. attorney prosecuted would-be presidential assassin John W. Hinckley Jr., agreed.

"We presented evidence that Hinckley stalked two presidents - Carter and Reagan. But that was completely ignored by the jury," diGenova said. "If there is evidence of deformity in the brain, which was the case with Hinckley, that could be a very serious problem for the government. . . . It sounds like [Loughner] was obviously psychotic, chemically imbalanced, and that's a legitimate defense. He surely seems unbalanced; whether that's a matter of law that he is insane is a question for the jury."

On the night before the shooting, Loughner rented a room at a Motel 6 near the railroad tracks on the western edge of Tucson. Using the room as a staging ground for a series of pre-dawn errands, Loughner drove back and forth several times between the motel and his home, where he used his computer for the last time, the sources said. At one point early in the morning, he posted a bulletin on his Myspace page titled "Goodbye friends," according to investigators.

Special Agent Jason Pack, an FBI spokesman, would not comment on the contents of Loughner's computer, which is in possession of the FBI. Jason Ogan, a spokesman for the Pima County Sheriff's Department, would not comment and said the sheriff would make no further comments.