WASHINGTON — Prosecutors are likely to seize on evidence that Jared Lee Loughner surfed the Internet for information on lethal injection and assassins in the hours before the Tucson shooting spree as evidence that he was not mentally incompetent, a federal law enforcement official said Thursday.

Loughner, who was indicted on five federal counts of attempted murder, including shooting Rep. Gabrielle Giffords, D-Ariz., was at his parents’ home on the computer reading information about the federal death penalty and profiles of famous assassins, the source said.

“”He was up late, the night before and into the morning hours,”” said the federal law enforcement official, who asked not to be identified because of the sensitivity of the case. “”

He added that Loughner’s Web traffic was downloaded on a disk and given to his defense attorneys to review when the two sides met on Monday inPhoenix for his arraignment. Loughner has pleaded not guilty.

The official said that Loughner’s computer research would tend to help rebut any attempts by the defense that he was so mentally ill that he could not reason that it was wrong to shoot the congresswoman, as well as six other people who died on Jan. 8 and 12 more who were injured.

“”It will go to his state-of-mind,”” the official said. “”It shows what he was thinking.””

More precisely, he said, it would suggest the 22-year-old Loughner intended to commit a federal crime by shooting a U.S. congresswoman. Although she has survived, also killed in the rampage was a federal judge who had gone to Giffords’ town hall meeting that morning at a Tucson parking lot.

Federal prosecutors have not yet filed any murder charges, but U.S. Attorney Dennis K. Burke indicated earlier this week that the government likely will ask for capital punishment, which is carried out by lethal injection. “”This case involves potential death penalty charges,”” Burke said.

Prosecutors are expected to seek a second, superseding indictment that includes the murder charges and, if Attorney General Eric H. Holder Jr. agrees, they will seek the death penalty.

“”They have to consult with Washington first,”” said a second law enforcement source, who asked for anonymity as well. “”That’s why the federal murder charges have been held back. But I think you will see a superseding indictment in due course. “”

State prosecutors in Tucson said they also are gathering evidence for their own case against Loughner, and they could seek the death penalty as well.