South Korea's state-run forensic lab said Friday it has failed to find out how a fugitive billionaire blamed for April's ferry disaster died.

Police said Tuesday that a badly decomposed body found in a rural area last month was identified as Yoo Byung-eun. Police cited DNA and fingerprint tests taken by the National Forensic Service. Authorities had sought the 73-year-old, believing he owned the sunken ferry and that his alleged corruption may have contributed to the disaster.

The forensic service said Friday that it has conducted additional tests on Yoo's body but couldn't discover the exact cause of his death because the body was decayed too much.

Agency chief Seo Joongseok and other forensic doctors told a televised news conference that they did not find any evidence showing that Yoo was poisoned, suffocated or died of external pressures or any disease. Police have said they haven't found any evidences that Yoo was murdered.

The discovery of Yoo's body fueled public criticism against the government following revelations authorities did not even suspect the body found June 12 could be Yoo's until recently and had still continued massive manhunts for him. Critics say authorities should have paid extra attention to the fact the body was found near one of Yoo's villas that they raided in May and that some items found near the body could have offered clues to his identity.

A high-level prosecutor resigned and two senior police officers were dismissed from their posts amid the criticism but South Korean media said higher-level officials must step down.

The government of President Park Geun-hye has already faced public indignation over its handling of the sinking, which left more than 300 people dead, mostly high school students travelling to a southern resort island.