Katō apparently posted messages from his mobile phone to a website called "Extreme Exchange, Revised", revealing his intentions through his final message twenty minutes before the attack. A police official stated the first message read, "I will kill people in Akihabara." Other messages he is alleged to have posted include, "If only I had a girlfriend, I wouldn't have quit work", "I would never have become addicted to my mobile phone. Anybody with hope couldn't possibly understand how I feel", and, "I don't have a single friend and I won't in the future. I'll be ignored because I'm ugly. I'm lower than trash because at least the trash gets recycled." The message also referred to "
a stabbing spree in Tsuchiura". Based on Katō's messages, commentators referred to the incident as another case of Japan's
hikikomori becoming more violent and suicidal. Later messages revealed Katō's plan to use a vehicle until it became inoperable, and then to use a knife to continue the attack on foot. He waited for Chūō-dōri to close at noon to vehicular traffic before commencing the attack.
