When Yoritomo died in 1199, Tokimasa and his family members attempted to increase their power and killed many senior vassals, including Kagetoki KAJIWARA, Yoshikazu HIKI (a maternal relative of MINAMOTO no Yoriie), OE no Hirotada and Tadatsune NITA.
1199年（正治元）、頼朝が死ぬと北条氏の権力強化を図って、有力御家人であった梶原景時や源頼家の外戚に当たる比企能員らを大江広元・仁田忠常らとともに殺害した。