Authorities in the city just south of Nuremberg have pieced together some 80 pages of evidence from the young man's laptop since the attack on September 17, senior public prosecutor Gudrun Lehnberger told journalists.
Letters to a “fictive female contact” revealed he felt excluded at school and had fears of failing his final exams or getting a grave illness.
“We don't know if this person exists,” Lehnberger said, adding that other evidence showed “he would have liked to have had a girlfriend, but did not have any success.”
The laptop documents also showed that he began specifically planning the attack in May, at some point listing the weapons he would come to use: an axe, knives and Molotov cocktails.
In July he decided to attack the third floor of the Carolinum Gymnasium – his college preparatory high school – because it had the highest concentration of classrooms in the building, investigators found. He wanted to kill as many students as possible and burn the school down.
“He did not seek out specific victims,” Lehnberger said.
He had a t-shirt printed to wear during the attack that read, “Made in School,” and believed he would be killed by police in the attack, though he also planned for suicide.
He also “emphatically” wrote that his parents were not to blame for what he would do.
On Thursday, the teen, who prosecutors said had been undergoing psychological counseling, attacked his school injuring nine students from the ninth and tenth grades and a teacher.
Two girls were severely injured. One suffered burns from a Molotov cocktail that hit her face, but she may return home soon, authorities said. The second girl was hit in the head with an axe several times, sustaining life-threatening skull and brain injuries. She will likely have further surgery.
The attacker was stopped after being shot by the police. He awoke on Monday, authorities said, but they have not yet issued a warrant due to his condition.
Investigators said they had questioned more than 60 witnesses, but the young man's parents declined to comment.