The suspect, identified as Atsunobu Usuda, 49, allegedly hurled multiple Molotov cocktails at the LDP building on Saturday before driving to the Prime Minister's Office, where he crashed into a fence and set his vehicle ablaze.
Police found numerous Molotov cocktails and about 20 jerricans of gasoline in Usuda's minivan. A search of his residence uncovered additional evidence, including glass bottles and a computer.
Usuda's motive remains unclear as he has remained silent during questioning.
His father told local media that Usuda had been involved in anti-nuclear activism and expressed dissatisfaction with Japan's election deposit system.
The incident came one week ahead of the Oct. 27 Lower House election.
Prime Minister Fumio Kishida, who was not present during the attack, said, "Democracy must never be allowed to succumb to violence."
The attack came after the assassination of former Prime Minister Shinzo Abe in July 2022 and an apparent attempt on Prime Minister Kishida's life in April 2023.
