What began as a judge’s inquiry into a $75 million attorney fee has morphed into hints of public corruption, with one of the top securities plaintiffs firms in the nation on the defensive.

A year ago, U.S. District Judge Mark Wolf in Boston began looking into potential overbilling in a securities class action settlement with State Street Corp. To spearhead the probe, Wolf brought in a special master who filed his 377-page report on May 14 under seal.