Portland Mayor Ted Wheeler's chief of staff Michael Cox has resigned, four sources in and close to City Hall tell WW.

Cox led the mayor's office for six months and been on Wheeler's staff for more than four years, including during his tenure as Oregon treasurer.

It was not immediately clear what triggered Cox's departure. The mayor faced blistering reviews of his first two years in office. Among other matters, Wheeler has faced criticism for the staffing of his office. Last month, Cox disclosed a relationship with a subordinate in the mayor's office—but Wheeler changed the organizational arrangement of his office to accommodate that relationship.

Wheeler's office and Cox did not immediately respond to a request for comment.

Update, 3 pm: 

The mayor has now offered a statement, without explanation for why Cox is leaving:

"I want to thank Michael Cox for his years of service. I've known him for many years. He is not only a trusted and loyal advisor but also a friend. Michael cares deeply about the progress and the future of the city. Kristin Dennis, who is my Deputy Chief of Staff, will be serving as Interim Chief of Staff."