Prosecutors say the 59-year-old Nathanson used the investor money to finance his failing Giant Golf Company and Play Big Enterprises, which sold clubs and accessories out of Irvine and Rancho Santa Margarita.
The money was also used for what prosecutors called "extravagant personal expenses, including gambling expenses and payments for three houses."
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Based on his pleas, Nathanson faces 120 years in prison. He is due to be sentenced on Feb. 9.