MILWAUKEE -

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.

A statement from Walker stated he's glad the investigation is complete.

"As many may remember, this entire matter began when we asked the District Attorney to look into concerns we had with respect to Operation Freedom.  We appreciate the effort that was undertaken and to bring appropriate matters to justice."

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