The Austrian identified by local media as Arthur A killed 10 people on Tuesday before shooting himself at his former high school in the southern city of Graz, sending shockwaves through Austria, which declared three days of national mourning.
Police found discarded plans for a bomb attack and a non-functional pipe bomb during a search of his home after the shooting in Graz, capital of the state of Styria.
Police are still trying to establish a motive, but Michael Lohnegger, head of Styria's criminal investigation office, said on Thursday findings indicated the man was very introverted and that his great passion was online first-person shooting games.
"He led an extremely withdrawn life; he didn't want to take part in activities in normal life outside in the real world, he preferred to withdraw into the virtual space," he told reporters.
Lohnegger said the man's closest friend had been questioned and that the shooter had social contacts with fellow online gamers. Police are checking whether he had assistance from other people in the run-up to the crime.
Police said the rampage at the school lasted about seven minutes and that the man shot himself in the head in a toilet about 10 minutes after beginning the attack.
A few minutes earlier, the man entered the school with a rucksack, then went into a bathroom to prepare for his assault.
Then he put on a weapon belt with a hunting knife, donned shooting glasses and a headset and armed himself with a Glock pistol and sawn-off shotgun, police said.
He opened fire at random on people on the second and third floors of the school, shooting off the locks to the doors in one of the classrooms he attacked, according to Lohnegger.
Most of the victims were apparently unknown to the shooter, but he did know one of the teachers who died, police said. It was unclear if that fact played a role in her death.
The shooter acquired the guns legally in April and May after passing a psychological evaluation for a necessary permit, and had practised shooting at a gun club since March, police said.
Authorities said the man failed to graduate at the school and Austrian newspaper Kronen Zeitung reported that after dropping out, he attempted to join the army but was deemed psychologically unsuitable due to his introversion.
Local media reported the man, who lived with his mother on the outskirts of Graz, felt bullied at the school and wanted revenge. Police declined to confirm this.
"There's no evidence from his private life that he ever expressed any anger or displeasure towards the school, pupils or teaching staff," Lohnegger said.
The man left behind a farewell note and video in which he apologised to his family for his actions and thanked them, but they gave no indication of his motives, police said.
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