The man who attacked and injured U.S. Ambassador to South Korea Mark Lippert on Thursday is a social activist with a history of violence and extreme forms of protest, South Korean media is reporting.

Thursday was not the first time Kim Ki-jong, 56, had  attacked diplomatic staff, having thrown two large cinder blocks at then-Japanese Ambassador to Seoul Toshinori Shigeie in 2010. While the Japanese ambassador was unharmed, his interpreter sustained light injuries.

Kim was sentenced to three years probation for the attack, which was motivated by the ongoing international dispute over the islets Korea calls Dokdo, Japan calls Takeshima