The handwritten note was in a notebook found under T.J. Solomon's bed after the May 20 shootings at Heritage, said Gary Nicholson, an agent of the Georgia Bureau of Investigation.
According to Nicholson, Solomon also wrote: "I am laughing at the victims who are getting down on their knees screaming, `Please, dear God, please don't let this crooked ... (obscenity deleted) murder me.'"
Testifying at a Juvenile Court hearing to decide whether Solomon should be tried as an adult, Nicholson said the note was found with three typewritten sheets of bomb-making instructions taken from the Internet.
Solomon is charged with shooting and wounding six classmates at Heritage High in the Atlanta suburb of Conyers on May 20, a month after the shooting at Columbine High in Littleton, Colo., in which 15 people were killed.
Nicholson quoted the note as saying: "One big question I leave behind for you to find is why. But for the sake of my brothers and sisters related to the Trench Coat Mafia, those answers will have to remain out of the public eye."
"This has nothing to do with Hitler, and it is not because I was picked on," the note said.
The two gunmen who killed 13 people and themselves in Colorado were members of the Trench Coat Mafia, a loosely knit group of students. According to diary entries, they considered themselves outcasts in the school.
District Attorney Richard Read argued in his opening statement that Solomon knew what he was doing.
"We will show that the interests of this child being kept in the juvenile system do not outweigh society's interests of being protected from someone who showed such callous disregard for human life," Read said.
Defense attorney Don Samuel argued in favor of keeping the case in juvenile court.
Judge William Schneider was not expected to rule immediately.
If Schneider keeps the case in juvenile court and Solomon is convicted, he could be sentenced to a maximum five years of detention. If convicted in Superior Court, Solomon would face 351 years in prison.
Nicholson said Solomon's note also referred to bombs at Heritage High, although no bombs were found.
"I'm not sure if bombing the school is a good idea," Nicholson read from the note. "If the cops are doing their jobs correctly, they'll find the bomb instructions along with this note.
"You'll find four of these bombs, including three placed in and around Heritage High School. All in all, 17 will be found."