An autopsy and DNA tests on the badly decomposed body of Yoo Byung-un revealed no evidence he was poisoned and there was also no indication of external trauma, forensic agency chief Seo Joong-seok told a news conference.
Mr Yoo, 73, was found dead in an orchard on June 12 after eluding authorities for nearly two months in South Korea's biggest manhunt, hiding behind the wall of a rural cabin in the final days while it was being searched.
Police identified his body only this week, 40 days after a farmer reported its discovery at his orchard.
This stirred a storm of public anger with what many said was incompetent work to arrest Yoo over the ferry disaster.
Lee Han-young, a senior official at the agency, said: "We did our best."