After the Battle of Serres, instead of a triumphal return, the way back to the Bulgarian capital ended tragically. Slightly before reaching Tarnovo, Ivan Asen I was murdered by his cousin Ivanko. The motive for this act is uncertain. Choniates stated, Ivanko wanted to rule "more justly and equitably" than Asan who had "governed everything by the sword". Stephenson concludes, Choniates' words show that Asen had introduced a "reign of terror", intimidating his subjects with the assistance of Cuman mercenaries. Vásáry, however, says the Byzantines encouraged Ivanko to kill Asen. Ivanko attempted to assume control in Tarnovo with Byzantine support, but Peter forced him to flee to the Byzantine Empire.