"It was impossible to identify the cause of Yoo Byung-eon's death as his body was badly decomposed," Xinhua quoted Seo Joong-seok, director of the National Forensic Service as saying.
The body, which was found on June 12 in the country's southern city of Suncheon, matched that of Yoo, but the death cause was not identified, the director said.
Police on Tuesday said that the body which was discovered 40 days ago in a plum field in Suncheon, around 415 km south of Seoul, was confirmed as that of Yoo through DNA testing and fingerprinting.
The police in Suncheon first believed it was the body of a homeless person due to its shabby appearance.
According to the toxicology test and autopsy conducted by the agency, no drugs and toxins were found on Yoo's body, which was also tested negative for alcohol infusion.
No signs were detected for being beaten or stabbed to death.
Whether he had been strangled to death could not be determined due to the deterioration of the body, the agency said.
Yoo, 73, owned Chonghaejin Marine, the operator of ferry Seowl which capsized and sank off the southwestern coast on April 16.
Yoo and his family have been suspected of committing a host of illegal acts such as embezzlement, dereliction of duty and tax evasion, which led to lack of funds in the ferry operator and caused lax safety practices such as the inveterate overloading.
A massive manhunt was launched after the ferry disaster as Yoo and his family dismissed prosecutors' summons several times and ran away.
Arrest warrants were issued for Yoo's second son and eldest daughter, who are staying abroad, and Yoo and his eldest son were placed on the most wanted list.