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South Korea has boosted to half a million dollars a reward for tips about the mysterious billionaire who prosecutors say owns a ferry that sank last month.

The ten-fold increase in the reward comes as officials struggle to find Yoo Byung-eun on allegations of tax evasion and embezzlement. Prosecutors said Monday that a failure to spend enough money on safety may be a reason for the April 16 sinking.

The sinking of the Sewol ferry left more than 300 people dead or missing, most of them high school students.

The $500,000 bounty is the same amount given if a South Korean citizen provides information about a North Korea-dispatched spy.

Police are also offering a bounty of $100,000, up from $30,000, for Yoo's eldest son. He faces embezzlement allegations.

Helicopters heading to aid passengers and crew aboard a South Korean ferry sinking off the island of Byungpoong in Jindo on April 16, 2014.SOUTH KOREA COAST GUARD / AFP - Getty Images

- Associated Press