Editing Tobias Rathjen (section)

===Ideology and beliefs===
Rathjen published a manifesto and videos on his personal website, showing his political beliefs and theories surrounding US President Donald Trump stealing his slogans, eugenics, and expressing frustration that he could never experience an intimate relationship with a woman due to psychological issues during his lifetime. He stated that he had been guided by voices inside his head since birth and that he was being followed by secret agents. Rathjen expressed a hatred for migrants, especially for people from the Middle East, Central Asia, Southeast Asia and North Africa. He also expressed a hatred for German citizens who allowed immigrants into their country, and considered them as "impure".

According to Germany's general prosecutor Peter Frank, Rathjen had contacted German authorities with his conspiracy theories three months before the attack: on 6 November 2019, Rathjen had written a letter to the Public Prosecutor General urging action against a "secret service" organisation, which he claimed was tapping into people's brains to control world events. He called on authorities to "approach me and communicate with me". No action was taken in response. Parts of this 19-page letter were virtually identical to his 24-page manifesto published on his website in February 2020, but it was unclear whether it included any threats against ethnic minorities.

Near the Arena Bar, text linked to the perpetrator's website was found written in graffiti on a wall before it was covered over by police.