Federal prosecutors have collected statements from some of billionaire Mark Walter’s most senior executives as authorities probe dealings at his insurance companies and the investment firm he runs, Guggenheim Partners LLC, according to people with knowledge of the matter. Daniel Towriss, the head of Walter’s insurance arm, and Andrew Rosenfield, Guggenheim’s president, are among executives who provided information over the past year, the people said, asking not to be identified discussing the sensitive investigation. Neither is thought to be a target of the probe, the people said.