MADISON, Wis. (AP) -

A secret investigation into Republican Wisconsin Gov. Scott Walker's former aides and associates has closed without any charges being filed against Walker.

Milwaukee County District Attorney John Chisholm announced Friday that the so-called John Doe investigation ended last week. Six people have been charged with crimes stemming from the investigation that looked into activity in the Milwaukee County executive's office when Walker held that position.

Four former Walker aides as well as a campaign donor have been convicted on charges ranging from theft to misconduct in office.

Walker's spokesman Cullen Werwie had no comment on the investigation closing and referred calls to Walker's campaign. A spokeswoman there did not immediately return calls.

Walker has long maintained he did nothing wrong and predicted months ago the investigation was nearing an end.