A 24-year-old man wearing a Joker costume attacked passengers on a Tokyo train line on Sunday evening.
As many as 17 people were injured as they headed for Halloween parties in the city.
Witnesses say the suspect was wearing a green shirt and purple suit.
He sprayed a clear liquid around the carriage and set it alight. Video footage showed passengers running through carriages away from the flames while others clambered through windows.
Media reports indicate that he was arrested immediately. Subsequent report stated that he had an affinity for the joker character.