(CNN) Two death-row inmates were hanged in Japan on Thursday, taking the number of executions in the country this year to 15 -- the highest annual total since 2008.

Keizo Okamoto, 60, a former senior member of a crime syndicate, and Hiroya Suemori, 67, a former investment adviser, had been sentenced to death in 2004 for a robbery-murder.

They stole about 100 million yen ($900,800) from the president of an investment advisory company in 1988 before killing him and another employee. They then packed the bodies into concrete.

"It is an extremely brutal case in which they robbed the precious life of the victims, indeed for selfish reasons," Justice Minister Takashi Yamashita said on Thursday.

Yamashita signed the order for their execution on Tuesday.

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