Conversation so far:
When did Steven Seagal get involved in martial arts?
Steven Seagal  started studying martial arts under Fumio Demura when he was a child.
What kind of martial arts was Steven Seagal doing?
Steven Seagal trained in aikido, karate, judo and kendo.

Question: who did he train wiht?
Self-contained rewrite: Who did Steven Seagal train with?