GRAZ, Austria (AP) — Investigators found a farewell letter and a non-functional pipe bomb when they searched the home of a man who opened fire at his former school in Austria, killing 10 people and taking his own life, police said Wednesday.

As Austria mourned the victims of what appeared to be the deadliest attack in its post-World War II history, with a national minute of silence planned Wednesday morning, questions remained about the motive of the shooter.

The 21-year-old Austrian man lived near Graz and was a student at the BORG Dreierschützengasse high school, in Austria's second-biggest city, who hadn't completed his studies. Police have said that he used two weapons, a shotgun and a handgun, which he appeared to have owned legally.

Police didn't elaborate on investigators' findings in a brief post on social network X. But a senior official who acknowledged that the letter had been found on Tuesday night said it hadn't allowed them to draw conclusions.

"A farewell letter in analog and digital form was found," Franz Ruf, the public security director at Austria's Interior Ministry, told ORF public television. "He says goodbye to his parents. But no motive can be inferred from the farewell letter, and that is a matter for further investigations."

Asked whether the assailant had attacked victims randomly or targeted them specifically, Ruf said that is also under investigation and he didn't want to speculate.

He said that wounded people were found on various levels of the school and, in one case, in front of the building.