Marty Blazer couldn’t believe what he had gotten himself into. By this summer, the Pittsburgh investment adviser had spent months crisscrossing the U.S., passing cash to coaches at some of the country’s top college basketball programs. None of them seemed to know he was secretly helping federal authorities build one of the biggest alleged bribery and corruption cases in college sports history.

“This is surreal stuff,” Mr. Blazer told a person familiar with the investigation. “They just keep sticking their hand out.”

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