After the incident in the Headmaster's office, Grindelwald was not heard from for many years. The deaths continued but there was never any evidence that would betray the perpetrator, though most steadfastly believed it was Grindelwald. Albus remained busy teaching Transfiguration and doing research, trying to locate the wizard.
By delving into books about Dark Magic long since forgotten, he learned much about Grindelwald's life. He read about his marriage to the psychotic witch Persephone Dialore and the horrific ending to her life. Much to his surprise, Albus found out that Grindelwald was not the wizard's surname. He had always found it odd that the only name the Headmaster was referred to by was Grindelwald and he had never heard a first name. As it turned out, Grindelwald McGonagall was a Scottish born son of the two leaders of a strange cult in the forests of Scotland. There was supposedly a large castle there where Grindelwald had lived as a child. Grindelwald had also killed his parents at the same castle.
It took Albus four years to discover all this information. He had not really found anything of importance until he found out Grindelwald's last name, and then many of the readings he came across were in a strange language that he spent months deciphering. As soon as he read about the castle, however, he had a feeling that it was the exact place he was looking for. As soon as the school year was over and Albus had the freedom to leave the castle, he set off for Scotland.