Alex Hribal, 16, was arraigned on 17 additional counts of attempted homicide stemming from the April 9 attack at Franklin Regional High School in Murrysville, east of Pittsburgh. He had been charged with four counts of attempted homicide and 21 counts of aggravated assault.
The criminal complaint submitted Friday also said police found a note in his locker that he signed and dated April 6 hinting at the attack, the Pittsburgh Post-Gazettereported.
"I can't wait to see the priceless and helpless looks on the faces of the students of one of the 'best schools in Pennsylvania' realize their precious lives are going to be taken by the only one among them that isn't a plebeian," the note read.
Detailing the new charges, police said the teen would not drop two 8-inch kitchen knives after being subdued by Vice Principal Sam King.
"No, I am not dropping them, my work is not done, I have more people to kill," he said, according to the complaint.
At least 12 victims were hospitalized with serious injuries, many with deep puncture wounds to the abdomen. Only two students remain hospitalized Friday, including 15-year-old Greg Keener, who is in critical condition.
Alex, who is being held in the Westmoreland County juvenile detention center, was described as a shy teen and a good student who showed no signs that pointed to such an attack.