A researcher at Two Sigma Investments adjusted the hedge fund’s investing models without authorization, the firm has told clients, leading to losses in some funds, big gains in others and fresh regulatory scrutiny.

The researcher, Jian Wu, a senior vice president at New York-based Two Sigma, was trying to boost his compensation, Two Sigma has told clients, without identifying Wu. He made changes over the past year that resulted in a total of $620 million in unexpected gains and losses, according to people close to the matter and investor letters. Two Sigma has placed Wu on administrative leave.