WASHINGTON —  The second-in-command at the government's top whistle-blower office has quit over concerns that the agency's mission and independence are at risk.

Deputy U.S. Special Counsel James Byrne is leaving amid an FBI investigation of his boss, Special Counsel Scott Bloch.

Bloch is accused of destroying evidence potentially showing he retaliated against his own staff.

Byrne's reasons for quitting are outlined in a July 10 letter to Bloch. The letter was obtained by The Associated Press.

In it, Byrne writes that the Office of Special Counsel needs a leader who does not put political agendas and personal vendettas ahead of its mission and employees.

A spokesman for Bloch declined comment.