A boyish 43 years old, Iftikar Ahmed ticked every box of the immigrant success story, going from Harvard Business School to Goldman Sachs Group Inc. and then landing as a partner at one of the oldest venture-capital firms in the country. He and his wife owned a mansion in Greenwich, Conn., and two apartments on Park Avenue in Manhattan, and gave large sums to local and Indian charities.

Yet before Mr. Ahmed fled the U.S. in May, he allegedly stole $65 million through a series of frauds that prosecutors and regulators said...